Regenerated code with changes

src/AddressBook/AddressBookProtos.cs

@@ -68,7 +68,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class Person : pb::GeneratedMessage<Person, Person.Builder> {
-    private static readonly Person defaultInstance = new Builder().BuildPartial();
+    private static readonly Person defaultInstance = new Person().MakeReadOnly();
     private static readonly string[] _personFieldNames = new string[] { "email", "id", "name", "phone" };
     private static readonly uint[] _personFieldTags = new uint[] { 26, 16, 10, 34 };
     public static Person DefaultInstance {
@@ -76,7 +76,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     }
     
     public override Person DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override Person ThisMessage {
@@ -108,7 +108,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class PhoneNumber : pb::GeneratedMessage<PhoneNumber, PhoneNumber.Builder> {
-        private static readonly PhoneNumber defaultInstance = new Builder().BuildPartial();
+        private static readonly PhoneNumber defaultInstance = new PhoneNumber().MakeReadOnly();
         private static readonly string[] _phoneNumberFieldNames = new string[] { "number", "type" };
         private static readonly uint[] _phoneNumberFieldTags = new uint[] { 10, 16 };
         public static PhoneNumber DefaultInstance {
@@ -116,7 +116,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         }
         
         public override PhoneNumber DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override PhoneNumber ThisMessage {
@@ -219,6 +219,10 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         public static PhoneNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private PhoneNumber MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -234,22 +238,22 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new PhoneNumber();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(PhoneNumber cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          PhoneNumber result;
+          private bool resultIsReadOnly;
+          private PhoneNumber result;
           
           private PhoneNumber PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               PhoneNumber original = result;
               result = new PhoneNumber();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -264,13 +268,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new PhoneNumber();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -286,11 +290,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }
           
           public override PhoneNumber BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessage other) {
@@ -555,6 +559,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     public static Person ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private Person MakeReadOnly() {
+      phone_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -570,22 +579,22 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new Person();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(Person cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      Person result;
+      private bool resultIsReadOnly;
+      private Person result;
       
       private Person PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           Person original = result;
           result = new Person();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -600,13 +609,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new Person();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -622,12 +631,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }
       
       public override Person BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.phone_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -838,7 +846,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class AddressBook : pb::GeneratedMessage<AddressBook, AddressBook.Builder> {
-    private static readonly AddressBook defaultInstance = new Builder().BuildPartial();
+    private static readonly AddressBook defaultInstance = new AddressBook().MakeReadOnly();
     private static readonly string[] _addressBookFieldNames = new string[] { "person" };
     private static readonly uint[] _addressBookFieldTags = new uint[] { 10 };
     public static AddressBook DefaultInstance {
@@ -846,7 +854,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     }
     
     public override AddressBook DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override AddressBook ThisMessage {
@@ -937,6 +945,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     public static AddressBook ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private AddressBook MakeReadOnly() {
+      person_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -952,22 +965,22 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new AddressBook();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(AddressBook cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      AddressBook result;
+      private bool resultIsReadOnly;
+      private AddressBook result;
       
       private AddressBook PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           AddressBook original = result;
           result = new AddressBook();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -982,13 +995,13 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new AddressBook();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1004,12 +1017,11 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }
       
       public override AddressBook BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.person_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs

@@ -60,7 +60,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class OptionsMessage : pb::GeneratedMessage<OptionsMessage, OptionsMessage.Builder> {
-    private static readonly OptionsMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly OptionsMessage defaultInstance = new OptionsMessage().MakeReadOnly();
     private static readonly string[] _optionsMessageFieldNames = new string[] { "customized", "normal", "options_message" };
     private static readonly uint[] _optionsMessageFieldTags = new uint[] { 26, 10, 18 };
     public static OptionsMessage DefaultInstance {
@@ -68,7 +68,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override OptionsMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override OptionsMessage ThisMessage {
@@ -186,6 +186,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static OptionsMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private OptionsMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -201,22 +205,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new OptionsMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(OptionsMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      OptionsMessage result;
+      private bool resultIsReadOnly;
+      private OptionsMessage result;
       
       private OptionsMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           OptionsMessage original = result;
           result = new OptionsMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -231,13 +235,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new OptionsMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -253,11 +257,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override OptionsMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs

@@ -64,7 +64,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestEmbedOptimizedForSize : pb::GeneratedMessage<TestEmbedOptimizedForSize, TestEmbedOptimizedForSize.Builder> {
-    private static readonly TestEmbedOptimizedForSize defaultInstance = new Builder().BuildPartial();
+    private static readonly TestEmbedOptimizedForSize defaultInstance = new TestEmbedOptimizedForSize().MakeReadOnly();
     private static readonly string[] _testEmbedOptimizedForSizeFieldNames = new string[] { "optional_message", "repeated_message" };
     private static readonly uint[] _testEmbedOptimizedForSizeFieldTags = new uint[] { 10, 18 };
     public static TestEmbedOptimizedForSize DefaultInstance {
@@ -72,7 +72,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestEmbedOptimizedForSize DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestEmbedOptimizedForSize ThisMessage {
@@ -89,12 +89,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int OptionalMessageFieldNumber = 1;
     private bool hasOptionalMessage;
-    private global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize optionalMessage_;
     public bool HasOptionalMessage {
       get { return hasOptionalMessage; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize OptionalMessage {
-      get { return optionalMessage_; }
+      get { return optionalMessage_ ?? global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance; }
     }
     
     public const int RepeatedMessageFieldNumber = 2;
@@ -182,6 +182,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestEmbedOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestEmbedOptimizedForSize MakeReadOnly() {
+      repeatedMessage_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -197,22 +202,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestEmbedOptimizedForSize();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestEmbedOptimizedForSize cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestEmbedOptimizedForSize result;
+      private bool resultIsReadOnly;
+      private TestEmbedOptimizedForSize result;
       
       private TestEmbedOptimizedForSize PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestEmbedOptimizedForSize original = result;
           result = new TestEmbedOptimizedForSize();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -227,13 +232,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestEmbedOptimizedForSize();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -249,12 +254,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestEmbedOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.repeatedMessage_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -377,7 +381,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearOptionalMessage() {
         PrepareBuilder();
         result.hasOptionalMessage = false;
-        result.optionalMessage_ = global::Google.ProtocolBuffers.TestProtos.TestOptimizedForSize.DefaultInstance;
+        result.optionalMessage_ = null;
         return this;
       }
       

src/ProtocolBuffers.Test/TestProtos/UnitTestExtrasIssuesProtoFile.cs

@@ -26,6 +26,10 @@ namespace UnitTest.Issues.TestProtos {
     internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.AB, global::UnitTest.Issues.TestProtos.AB.Builder> internal__static_unittest_issues_AB__FieldAccessorTable;
     internal static pbd::MessageDescriptor internal__static_unittest_issues_NumberField__Descriptor;
     internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.NumberField, global::UnitTest.Issues.TestProtos.NumberField.Builder> internal__static_unittest_issues_NumberField__FieldAccessorTable;
+    internal static pbd::MessageDescriptor internal__static_unittest_issues_MyMessageAReferenceB__Descriptor;
+    internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageAReferenceB, global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder> internal__static_unittest_issues_MyMessageAReferenceB__FieldAccessorTable;
+    internal static pbd::MessageDescriptor internal__static_unittest_issues_MyMessageBReferenceA__Descriptor;
+    internal static pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageBReferenceA, global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder> internal__static_unittest_issues_MyMessageBReferenceA__FieldAccessorTable;
     #endregion
     #region Descriptor
     public static pbd::FileDescriptor Descriptor {
@@ -38,9 +42,12 @@ namespace UnitTest.Issues.TestProtos {
           "ChxleHRlc3QvdW5pdHRlc3RfaXNzdWVzLnByb3RvEg91bml0dGVzdF9pc3N1" + 
           "ZXMaJGdvb2dsZS9wcm90b2J1Zi9jc2hhcnBfb3B0aW9ucy5wcm90byIPCgFB" + 
           "EgoKAl9BGAEgASgFIg8KAUISCgoCQl8YASABKAUiEQoCQUISCwoDYV9iGAEg" + 
-          "ASgFIhoKC051bWJlckZpZWxkEgsKA18wMRgBIAEoBUJASAHCPjsKGlVuaXRU" + 
-          "ZXN0Lklzc3Vlcy5UZXN0UHJvdG9zEh1Vbml0VGVzdEV4dHJhc0lzc3Vlc1By" + 
-          "b3RvRmlsZQ==");
+          "ASgFIhoKC051bWJlckZpZWxkEgsKA18wMRgBIAEoBSJMChRNeU1lc3NhZ2VB" + 
+          "UmVmZXJlbmNlQhI0CgV2YWx1ZRgBIAIoCzIlLnVuaXR0ZXN0X2lzc3Vlcy5N" + 
+          "eU1lc3NhZ2VCUmVmZXJlbmNlQSJMChRNeU1lc3NhZ2VCUmVmZXJlbmNlQRI0" + 
+          "CgV2YWx1ZRgBIAIoCzIlLnVuaXR0ZXN0X2lzc3Vlcy5NeU1lc3NhZ2VBUmVm" + 
+          "ZXJlbmNlQkJASAHCPjsKGlVuaXRUZXN0Lklzc3Vlcy5UZXN0UHJvdG9zEh1V" + 
+          "bml0VGVzdEV4dHJhc0lzc3Vlc1Byb3RvRmlsZQ==");
       pbd::FileDescriptor.InternalDescriptorAssigner assigner = delegate(pbd::FileDescriptor root) {
         descriptor = root;
         internal__static_unittest_issues_A__Descriptor = Descriptor.MessageTypes[0];
@@ -59,6 +66,14 @@ namespace UnitTest.Issues.TestProtos {
         internal__static_unittest_issues_NumberField__FieldAccessorTable = 
             new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.NumberField, global::UnitTest.Issues.TestProtos.NumberField.Builder>(internal__static_unittest_issues_NumberField__Descriptor,
                 new string[] { "_01", });
+        internal__static_unittest_issues_MyMessageAReferenceB__Descriptor = Descriptor.MessageTypes[4];
+        internal__static_unittest_issues_MyMessageAReferenceB__FieldAccessorTable = 
+            new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageAReferenceB, global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder>(internal__static_unittest_issues_MyMessageAReferenceB__Descriptor,
+                new string[] { "Value", });
+        internal__static_unittest_issues_MyMessageBReferenceA__Descriptor = Descriptor.MessageTypes[5];
+        internal__static_unittest_issues_MyMessageBReferenceA__FieldAccessorTable = 
+            new pb::FieldAccess.FieldAccessorTable<global::UnitTest.Issues.TestProtos.MyMessageBReferenceA, global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder>(internal__static_unittest_issues_MyMessageBReferenceA__Descriptor,
+                new string[] { "Value", });
         pb::ExtensionRegistry registry = pb::ExtensionRegistry.CreateInstance();
         RegisterAllExtensions(registry);
         global::Google.ProtocolBuffers.DescriptorProtos.CSharpOptions.RegisterAllExtensions(registry);
@@ -77,7 +92,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class A : pb::GeneratedMessage<A, A.Builder> {
-    private static readonly A defaultInstance = new Builder().BuildPartial();
+    private static readonly A defaultInstance = new A().MakeReadOnly();
     private static readonly string[] _aFieldNames = new string[] { "_A" };
     private static readonly uint[] _aFieldTags = new uint[] { 8 };
     public static A DefaultInstance {
@@ -85,7 +100,7 @@ namespace UnitTest.Issues.TestProtos {
     }
     
     public override A DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override A ThisMessage {
@@ -171,6 +186,10 @@ namespace UnitTest.Issues.TestProtos {
     public static A ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private A MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -186,22 +205,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new A();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(A cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      A result;
+      private bool resultIsReadOnly;
+      private A result;
       
       private A PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           A original = result;
           result = new A();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -216,13 +235,13 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new A();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -238,11 +257,11 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override A BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -346,7 +365,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class B : pb::GeneratedMessage<B, B.Builder> {
-    private static readonly B defaultInstance = new Builder().BuildPartial();
+    private static readonly B defaultInstance = new B().MakeReadOnly();
     private static readonly string[] _bFieldNames = new string[] { "B_" };
     private static readonly uint[] _bFieldTags = new uint[] { 8 };
     public static B DefaultInstance {
@@ -354,7 +373,7 @@ namespace UnitTest.Issues.TestProtos {
     }
     
     public override B DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override B ThisMessage {
@@ -440,6 +459,10 @@ namespace UnitTest.Issues.TestProtos {
     public static B ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private B MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -455,22 +478,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new B();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(B cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      B result;
+      private bool resultIsReadOnly;
+      private B result;
       
       private B PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           B original = result;
           result = new B();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -485,13 +508,13 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new B();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -507,11 +530,11 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override B BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -615,7 +638,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class AB : pb::GeneratedMessage<AB, AB.Builder> {
-    private static readonly AB defaultInstance = new Builder().BuildPartial();
+    private static readonly AB defaultInstance = new AB().MakeReadOnly();
     private static readonly string[] _aBFieldNames = new string[] { "a_b" };
     private static readonly uint[] _aBFieldTags = new uint[] { 8 };
     public static AB DefaultInstance {
@@ -623,7 +646,7 @@ namespace UnitTest.Issues.TestProtos {
     }
     
     public override AB DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override AB ThisMessage {
@@ -709,6 +732,10 @@ namespace UnitTest.Issues.TestProtos {
     public static AB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private AB MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -724,22 +751,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new AB();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(AB cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      AB result;
+      private bool resultIsReadOnly;
+      private AB result;
       
       private AB PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           AB original = result;
           result = new AB();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -754,13 +781,13 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new AB();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -776,11 +803,11 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override AB BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -884,7 +911,7 @@ namespace UnitTest.Issues.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class NumberField : pb::GeneratedMessage<NumberField, NumberField.Builder> {
-    private static readonly NumberField defaultInstance = new Builder().BuildPartial();
+    private static readonly NumberField defaultInstance = new NumberField().MakeReadOnly();
     private static readonly string[] _numberFieldFieldNames = new string[] { "_01" };
     private static readonly uint[] _numberFieldFieldTags = new uint[] { 8 };
     public static NumberField DefaultInstance {
@@ -892,7 +919,7 @@ namespace UnitTest.Issues.TestProtos {
     }
     
     public override NumberField DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override NumberField ThisMessage {
@@ -980,6 +1007,10 @@ namespace UnitTest.Issues.TestProtos {
     public static NumberField ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private NumberField MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -995,22 +1026,22 @@ namespace UnitTest.Issues.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new NumberField();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(NumberField cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      NumberField result;
+      private bool resultIsReadOnly;
+      private NumberField result;
       
       private NumberField PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           NumberField original = result;
           result = new NumberField();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1025,13 +1056,13 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new NumberField();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1047,11 +1078,11 @@ namespace UnitTest.Issues.TestProtos {
       }
       
       public override NumberField BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -1153,6 +1184,606 @@ namespace UnitTest.Issues.TestProtos {
     }
   }
   
+  [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+  [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+  [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
+  public sealed partial class MyMessageAReferenceB : pb::GeneratedMessage<MyMessageAReferenceB, MyMessageAReferenceB.Builder> {
+    private static readonly MyMessageAReferenceB defaultInstance = new MyMessageAReferenceB().MakeReadOnly();
+    private static readonly string[] _myMessageAReferenceBFieldNames = new string[] { "value" };
+    private static readonly uint[] _myMessageAReferenceBFieldTags = new uint[] { 10 };
+    public static MyMessageAReferenceB DefaultInstance {
+      get { return defaultInstance; }
+    }
+    
+    public override MyMessageAReferenceB DefaultInstanceForType {
+      get { return DefaultInstance; }
+    }
+    
+    protected override MyMessageAReferenceB ThisMessage {
+      get { return this; }
+    }
+    
+    public static pbd::MessageDescriptor Descriptor {
+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageAReferenceB__Descriptor; }
+    }
+    
+    protected override pb::FieldAccess.FieldAccessorTable<MyMessageAReferenceB, MyMessageAReferenceB.Builder> InternalFieldAccessors {
+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageAReferenceB__FieldAccessorTable; }
+    }
+    
+    public const int ValueFieldNumber = 1;
+    private bool hasValue;
+    private global::UnitTest.Issues.TestProtos.MyMessageBReferenceA value_;
+    public bool HasValue {
+      get { return hasValue; }
+    }
+    public global::UnitTest.Issues.TestProtos.MyMessageBReferenceA Value {
+      get { return value_ ?? global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance; }
+    }
+    
+    public override bool IsInitialized {
+      get {
+        if (!hasValue) return false;
+        if (!Value.IsInitialized) return false;
+        return true;
+      }
+    }
+    
+    public override void WriteTo(pb::ICodedOutputStream output) {
+      int size = SerializedSize;
+      string[] field_names = _myMessageAReferenceBFieldNames;
+      if (hasValue) {
+        output.WriteMessage(1, field_names[0], Value);
+      }
+      UnknownFields.WriteTo(output);
+    }
+    
+    private int memoizedSerializedSize = -1;
+    public override int SerializedSize {
+      get {
+        int size = memoizedSerializedSize;
+        if (size != -1) return size;
+        
+        size = 0;
+        if (hasValue) {
+          size += pb::CodedOutputStream.ComputeMessageSize(1, Value);
+        }
+        size += UnknownFields.SerializedSize;
+        memoizedSerializedSize = size;
+        return size;
+      }
+    }
+    
+    public static MyMessageAReferenceB ParseFrom(pb::ByteString data) {
+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(pb::ByteString data, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(byte[] data) {
+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(byte[] data, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(global::System.IO.Stream input) {
+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseDelimitedFrom(global::System.IO.Stream input) {
+      return CreateBuilder().MergeDelimitedFrom(input).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseDelimitedFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {
+      return CreateBuilder().MergeDelimitedFrom(input, extensionRegistry).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(pb::ICodedInputStream input) {
+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();
+    }
+    public static MyMessageAReferenceB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
+    }
+    private MyMessageAReferenceB MakeReadOnly() {
+      return this;
+    }
+    
+    public static Builder CreateBuilder() { return new Builder(); }
+    public override Builder ToBuilder() { return CreateBuilder(this); }
+    public override Builder CreateBuilderForType() { return new Builder(); }
+    public static Builder CreateBuilder(MyMessageAReferenceB prototype) {
+      return new Builder(prototype);
+    }
+    
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
+    public sealed partial class Builder : pb::GeneratedBuilder<MyMessageAReferenceB, Builder> {
+      protected override Builder ThisBuilder {
+        get { return this; }
+      }
+      public Builder() {
+        result = DefaultInstance;
+        resultIsReadOnly = true;
+      }
+      internal Builder(MyMessageAReferenceB cloneFrom) {
+        result = cloneFrom;
+        resultIsReadOnly = true;
+      }
+      
+      private bool resultIsReadOnly;
+      private MyMessageAReferenceB result;
+      
+      private MyMessageAReferenceB PrepareBuilder() {
+        if (resultIsReadOnly) {
+          MyMessageAReferenceB original = result;
+          result = new MyMessageAReferenceB();
+          resultIsReadOnly = false;
+          MergeFrom(original);
+        }
+        return result;
+      }
+      
+      public override bool IsInitialized {
+        get { return result.IsInitialized; }
+      }
+      
+      protected override MyMessageAReferenceB MessageBeingBuilt {
+        get { return PrepareBuilder(); }
+      }
+      
+      public override Builder Clear() {
+        result = DefaultInstance;
+        resultIsReadOnly = true;
+        return this;
+      }
+      
+      public override Builder Clone() {
+        if (resultIsReadOnly) {
+          return new Builder(result);
+        } else {
+          return new Builder().MergeFrom(result);
+        }
+      }
+      
+      public override pbd::MessageDescriptor DescriptorForType {
+        get { return global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Descriptor; }
+      }
+      
+      public override MyMessageAReferenceB DefaultInstanceForType {
+        get { return global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance; }
+      }
+      
+      public override MyMessageAReferenceB BuildPartial() {
+        if (resultIsReadOnly) {
+          return result;
+        }
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
+      }
+      
+      public override Builder MergeFrom(pb::IMessage other) {
+        if (other is MyMessageAReferenceB) {
+          return MergeFrom((MyMessageAReferenceB) other);
+        } else {
+          base.MergeFrom(other);
+          return this;
+        }
+      }
+      
+      public override Builder MergeFrom(MyMessageAReferenceB other) {
+        if (other == global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance) return this;
+        PrepareBuilder();
+        if (other.HasValue) {
+          MergeValue(other.Value);
+        }
+        this.MergeUnknownFields(other.UnknownFields);
+        return this;
+      }
+      
+      public override Builder MergeFrom(pb::ICodedInputStream input) {
+        return MergeFrom(input, pb::ExtensionRegistry.Empty);
+      }
+      
+      public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
+        PrepareBuilder();
+        pb::UnknownFieldSet.Builder unknownFields = null;
+        uint tag;
+        string field_name;
+        while (input.ReadTag(out tag, out field_name)) {
+          if(tag == 0 && field_name != null) {
+            int field_ordinal = global::System.Array.BinarySearch(_myMessageAReferenceBFieldNames, field_name, global::System.StringComparer.Ordinal);
+            if(field_ordinal >= 0)
+              tag = _myMessageAReferenceBFieldTags[field_ordinal];
+            else {
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);
+              continue;
+            }
+          }
+          switch (tag) {
+            case 0: {
+              throw pb::InvalidProtocolBufferException.InvalidTag();
+            }
+            default: {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
+                  this.UnknownFields = unknownFields.Build();
+                }
+                return this;
+              }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);
+              break;
+            }
+            case 10: {
+              global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder subBuilder = global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.CreateBuilder();
+              if (result.hasValue) {
+                subBuilder.MergeFrom(Value);
+              }
+              input.ReadMessage(subBuilder, extensionRegistry);
+              Value = subBuilder.BuildPartial();
+              break;
+            }
+          }
+        }
+        
+        if (unknownFields != null) {
+          this.UnknownFields = unknownFields.Build();
+        }
+        return this;
+      }
+      
+      
+      public bool HasValue {
+       get { return result.hasValue; }
+      }
+      public global::UnitTest.Issues.TestProtos.MyMessageBReferenceA Value {
+        get { return result.Value; }
+        set { SetValue(value); }
+      }
+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageBReferenceA value) {
+        pb::ThrowHelper.ThrowIfNull(value, "value");
+        PrepareBuilder();
+        result.hasValue = true;
+        result.value_ = value;
+        return this;
+      }
+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Builder builderForValue) {
+        pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");
+        PrepareBuilder();
+        result.hasValue = true;
+        result.value_ = builderForValue.Build();
+        return this;
+      }
+      public Builder MergeValue(global::UnitTest.Issues.TestProtos.MyMessageBReferenceA value) {
+        pb::ThrowHelper.ThrowIfNull(value, "value");
+        PrepareBuilder();
+        if (result.hasValue &&
+            result.value_ != global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance) {
+            result.value_ = global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.CreateBuilder(result.value_).MergeFrom(value).BuildPartial();
+        } else {
+          result.value_ = value;
+        }
+        result.hasValue = true;
+        return this;
+      }
+      public Builder ClearValue() {
+        PrepareBuilder();
+        result.hasValue = false;
+        result.value_ = null;
+        return this;
+      }
+    }
+    static MyMessageAReferenceB() {
+      object.ReferenceEquals(global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.Descriptor, null);
+    }
+  }
+  
+  [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+  [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+  [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
+  public sealed partial class MyMessageBReferenceA : pb::GeneratedMessage<MyMessageBReferenceA, MyMessageBReferenceA.Builder> {
+    private static readonly MyMessageBReferenceA defaultInstance = new MyMessageBReferenceA().MakeReadOnly();
+    private static readonly string[] _myMessageBReferenceAFieldNames = new string[] { "value" };
+    private static readonly uint[] _myMessageBReferenceAFieldTags = new uint[] { 10 };
+    public static MyMessageBReferenceA DefaultInstance {
+      get { return defaultInstance; }
+    }
+    
+    public override MyMessageBReferenceA DefaultInstanceForType {
+      get { return DefaultInstance; }
+    }
+    
+    protected override MyMessageBReferenceA ThisMessage {
+      get { return this; }
+    }
+    
+    public static pbd::MessageDescriptor Descriptor {
+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageBReferenceA__Descriptor; }
+    }
+    
+    protected override pb::FieldAccess.FieldAccessorTable<MyMessageBReferenceA, MyMessageBReferenceA.Builder> InternalFieldAccessors {
+      get { return global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.internal__static_unittest_issues_MyMessageBReferenceA__FieldAccessorTable; }
+    }
+    
+    public const int ValueFieldNumber = 1;
+    private bool hasValue;
+    private global::UnitTest.Issues.TestProtos.MyMessageAReferenceB value_;
+    public bool HasValue {
+      get { return hasValue; }
+    }
+    public global::UnitTest.Issues.TestProtos.MyMessageAReferenceB Value {
+      get { return value_ ?? global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance; }
+    }
+    
+    public override bool IsInitialized {
+      get {
+        if (!hasValue) return false;
+        if (!Value.IsInitialized) return false;
+        return true;
+      }
+    }
+    
+    public override void WriteTo(pb::ICodedOutputStream output) {
+      int size = SerializedSize;
+      string[] field_names = _myMessageBReferenceAFieldNames;
+      if (hasValue) {
+        output.WriteMessage(1, field_names[0], Value);
+      }
+      UnknownFields.WriteTo(output);
+    }
+    
+    private int memoizedSerializedSize = -1;
+    public override int SerializedSize {
+      get {
+        int size = memoizedSerializedSize;
+        if (size != -1) return size;
+        
+        size = 0;
+        if (hasValue) {
+          size += pb::CodedOutputStream.ComputeMessageSize(1, Value);
+        }
+        size += UnknownFields.SerializedSize;
+        memoizedSerializedSize = size;
+        return size;
+      }
+    }
+    
+    public static MyMessageBReferenceA ParseFrom(pb::ByteString data) {
+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(pb::ByteString data, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(byte[] data) {
+      return ((Builder) CreateBuilder().MergeFrom(data)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(byte[] data, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(data, extensionRegistry)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(global::System.IO.Stream input) {
+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseDelimitedFrom(global::System.IO.Stream input) {
+      return CreateBuilder().MergeDelimitedFrom(input).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseDelimitedFrom(global::System.IO.Stream input, pb::ExtensionRegistry extensionRegistry) {
+      return CreateBuilder().MergeDelimitedFrom(input, extensionRegistry).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(pb::ICodedInputStream input) {
+      return ((Builder) CreateBuilder().MergeFrom(input)).BuildParsed();
+    }
+    public static MyMessageBReferenceA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
+      return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
+    }
+    private MyMessageBReferenceA MakeReadOnly() {
+      return this;
+    }
+    
+    public static Builder CreateBuilder() { return new Builder(); }
+    public override Builder ToBuilder() { return CreateBuilder(this); }
+    public override Builder CreateBuilderForType() { return new Builder(); }
+    public static Builder CreateBuilder(MyMessageBReferenceA prototype) {
+      return new Builder(prototype);
+    }
+    
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
+    public sealed partial class Builder : pb::GeneratedBuilder<MyMessageBReferenceA, Builder> {
+      protected override Builder ThisBuilder {
+        get { return this; }
+      }
+      public Builder() {
+        result = DefaultInstance;
+        resultIsReadOnly = true;
+      }
+      internal Builder(MyMessageBReferenceA cloneFrom) {
+        result = cloneFrom;
+        resultIsReadOnly = true;
+      }
+      
+      private bool resultIsReadOnly;
+      private MyMessageBReferenceA result;
+      
+      private MyMessageBReferenceA PrepareBuilder() {
+        if (resultIsReadOnly) {
+          MyMessageBReferenceA original = result;
+          result = new MyMessageBReferenceA();
+          resultIsReadOnly = false;
+          MergeFrom(original);
+        }
+        return result;
+      }
+      
+      public override bool IsInitialized {
+        get { return result.IsInitialized; }
+      }
+      
+      protected override MyMessageBReferenceA MessageBeingBuilt {
+        get { return PrepareBuilder(); }
+      }
+      
+      public override Builder Clear() {
+        result = DefaultInstance;
+        resultIsReadOnly = true;
+        return this;
+      }
+      
+      public override Builder Clone() {
+        if (resultIsReadOnly) {
+          return new Builder(result);
+        } else {
+          return new Builder().MergeFrom(result);
+        }
+      }
+      
+      public override pbd::MessageDescriptor DescriptorForType {
+        get { return global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.Descriptor; }
+      }
+      
+      public override MyMessageBReferenceA DefaultInstanceForType {
+        get { return global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance; }
+      }
+      
+      public override MyMessageBReferenceA BuildPartial() {
+        if (resultIsReadOnly) {
+          return result;
+        }
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
+      }
+      
+      public override Builder MergeFrom(pb::IMessage other) {
+        if (other is MyMessageBReferenceA) {
+          return MergeFrom((MyMessageBReferenceA) other);
+        } else {
+          base.MergeFrom(other);
+          return this;
+        }
+      }
+      
+      public override Builder MergeFrom(MyMessageBReferenceA other) {
+        if (other == global::UnitTest.Issues.TestProtos.MyMessageBReferenceA.DefaultInstance) return this;
+        PrepareBuilder();
+        if (other.HasValue) {
+          MergeValue(other.Value);
+        }
+        this.MergeUnknownFields(other.UnknownFields);
+        return this;
+      }
+      
+      public override Builder MergeFrom(pb::ICodedInputStream input) {
+        return MergeFrom(input, pb::ExtensionRegistry.Empty);
+      }
+      
+      public override Builder MergeFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
+        PrepareBuilder();
+        pb::UnknownFieldSet.Builder unknownFields = null;
+        uint tag;
+        string field_name;
+        while (input.ReadTag(out tag, out field_name)) {
+          if(tag == 0 && field_name != null) {
+            int field_ordinal = global::System.Array.BinarySearch(_myMessageBReferenceAFieldNames, field_name, global::System.StringComparer.Ordinal);
+            if(field_ordinal >= 0)
+              tag = _myMessageBReferenceAFieldTags[field_ordinal];
+            else {
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);
+              continue;
+            }
+          }
+          switch (tag) {
+            case 0: {
+              throw pb::InvalidProtocolBufferException.InvalidTag();
+            }
+            default: {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
+                  this.UnknownFields = unknownFields.Build();
+                }
+                return this;
+              }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag, field_name);
+              break;
+            }
+            case 10: {
+              global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder subBuilder = global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.CreateBuilder();
+              if (result.hasValue) {
+                subBuilder.MergeFrom(Value);
+              }
+              input.ReadMessage(subBuilder, extensionRegistry);
+              Value = subBuilder.BuildPartial();
+              break;
+            }
+          }
+        }
+        
+        if (unknownFields != null) {
+          this.UnknownFields = unknownFields.Build();
+        }
+        return this;
+      }
+      
+      
+      public bool HasValue {
+       get { return result.hasValue; }
+      }
+      public global::UnitTest.Issues.TestProtos.MyMessageAReferenceB Value {
+        get { return result.Value; }
+        set { SetValue(value); }
+      }
+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageAReferenceB value) {
+        pb::ThrowHelper.ThrowIfNull(value, "value");
+        PrepareBuilder();
+        result.hasValue = true;
+        result.value_ = value;
+        return this;
+      }
+      public Builder SetValue(global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.Builder builderForValue) {
+        pb::ThrowHelper.ThrowIfNull(builderForValue, "builderForValue");
+        PrepareBuilder();
+        result.hasValue = true;
+        result.value_ = builderForValue.Build();
+        return this;
+      }
+      public Builder MergeValue(global::UnitTest.Issues.TestProtos.MyMessageAReferenceB value) {
+        pb::ThrowHelper.ThrowIfNull(value, "value");
+        PrepareBuilder();
+        if (result.hasValue &&
+            result.value_ != global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.DefaultInstance) {
+            result.value_ = global::UnitTest.Issues.TestProtos.MyMessageAReferenceB.CreateBuilder(result.value_).MergeFrom(value).BuildPartial();
+        } else {
+          result.value_ = value;
+        }
+        result.hasValue = true;
+        return this;
+      }
+      public Builder ClearValue() {
+        PrepareBuilder();
+        result.hasValue = false;
+        result.value_ = null;
+        return this;
+      }
+    }
+    static MyMessageBReferenceA() {
+      object.ReferenceEquals(global::UnitTest.Issues.TestProtos.UnitTestExtrasIssuesProtoFile.Descriptor, null);
+    }
+  }
+  
   #endregion
   
 }

src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSizeProtoFile.cs

@@ -137,13 +137,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage1 : pb::GeneratedMessage<SizeMessage1, SizeMessage1.Builder> {
-    private static readonly SizeMessage1 defaultInstance = new Builder().BuildPartial();
+    private static readonly SizeMessage1 defaultInstance = new SizeMessage1().MakeReadOnly();
     public static SizeMessage1 DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override SizeMessage1 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SizeMessage1 ThisMessage {
@@ -504,12 +504,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int Field15FieldNumber = 15;
     private bool hasField15;
-    private global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage field15_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage field15_;
     public bool HasField15 {
       get { return hasField15; }
     }
     public global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage Field15 {
-      get { return field15_; }
+      get { return field15_ ?? global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance; }
     }
     
     public const int Field78FieldNumber = 78;
@@ -602,6 +602,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SizeMessage1 MakeReadOnly() {
+      field5_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -617,22 +622,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SizeMessage1();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SizeMessage1 cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SizeMessage1 result;
+      private bool resultIsReadOnly;
+      private SizeMessage1 result;
       
       private SizeMessage1 PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SizeMessage1 original = result;
           result = new SizeMessage1();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -647,13 +652,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SizeMessage1();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -669,12 +674,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SizeMessage1 BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.field5_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -1416,7 +1420,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearField15() {
         PrepareBuilder();
         result.hasField15 = false;
-        result.field15_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance;
+        result.field15_ = null;
         return this;
       }
       
@@ -1550,13 +1554,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage1SubMessage : pb::GeneratedMessage<SizeMessage1SubMessage, SizeMessage1SubMessage.Builder> {
-    private static readonly SizeMessage1SubMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly SizeMessage1SubMessage defaultInstance = new SizeMessage1SubMessage().MakeReadOnly();
     public static SizeMessage1SubMessage DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override SizeMessage1SubMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SizeMessage1SubMessage ThisMessage {
@@ -1805,6 +1809,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage1SubMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SizeMessage1SubMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1820,22 +1828,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SizeMessage1SubMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SizeMessage1SubMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SizeMessage1SubMessage result;
+      private bool resultIsReadOnly;
+      private SizeMessage1SubMessage result;
       
       private SizeMessage1SubMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SizeMessage1SubMessage original = result;
           result = new SizeMessage1SubMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1850,13 +1858,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SizeMessage1SubMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1872,11 +1880,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SizeMessage1SubMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -2299,13 +2307,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage2 : pb::GeneratedMessage<SizeMessage2, SizeMessage2.Builder> {
-    private static readonly SizeMessage2 defaultInstance = new Builder().BuildPartial();
+    private static readonly SizeMessage2 defaultInstance = new SizeMessage2().MakeReadOnly();
     public static SizeMessage2 DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override SizeMessage2 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SizeMessage2 ThisMessage {
@@ -2329,13 +2337,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Group1 : pb::GeneratedMessage<Group1, Group1.Builder> {
-        private static readonly Group1 defaultInstance = new Builder().BuildPartial();
+        private static readonly Group1 defaultInstance = new Group1().MakeReadOnly();
         public static Group1 DefaultInstance {
           get { return defaultInstance; }
         }
         
         public override Group1 DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Group1 ThisMessage {
@@ -2509,12 +2517,12 @@ namespace Google.ProtocolBuffers.TestProtos {
         
         public const int Field31FieldNumber = 31;
         private bool hasField31;
-        private global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance;
+        private global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage field31_;
         public bool HasField31 {
           get { return hasField31; }
         }
         public global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage Field31 {
-          get { return field31_; }
+          get { return field31_ ?? global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance; }
         }
         
         public static Group1 ParseFrom(pb::ByteString data) {
@@ -2547,6 +2555,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Group1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private Group1 MakeReadOnly() {
+          field14_.MakeReadOnly();
+          field22_.MakeReadOnly();
+          field73_.MakeReadOnly();
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -2562,22 +2577,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new Group1();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(Group1 cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          Group1 result;
+          private bool resultIsReadOnly;
+          private Group1 result;
           
           private Group1 PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               Group1 original = result;
               result = new Group1();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -2592,13 +2607,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new Group1();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -2614,14 +2629,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Group1 BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            result.field14_.MakeReadOnly();
-            result.field22_.MakeReadOnly();
-            result.field73_.MakeReadOnly();
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -3003,7 +3015,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           public Builder ClearField31() {
             PrepareBuilder();
             result.hasField31 = false;
-            result.field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance;
+            result.field31_ = null;
             return this;
           }
         }
@@ -3353,6 +3365,14 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SizeMessage2 MakeReadOnly() {
+      group1_.MakeReadOnly();
+      field128_.MakeReadOnly();
+      field127_.MakeReadOnly();
+      field130_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -3368,22 +3388,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SizeMessage2();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SizeMessage2 cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SizeMessage2 result;
+      private bool resultIsReadOnly;
+      private SizeMessage2 result;
       
       private SizeMessage2 PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SizeMessage2 original = result;
           result = new SizeMessage2();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -3398,13 +3418,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SizeMessage2();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -3420,15 +3440,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SizeMessage2 BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.group1_.MakeReadOnly();
-        result.field128_.MakeReadOnly();
-        result.field127_.MakeReadOnly();
-        result.field130_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -4102,13 +4118,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage2GroupedMessage : pb::GeneratedMessage<SizeMessage2GroupedMessage, SizeMessage2GroupedMessage.Builder> {
-    private static readonly SizeMessage2GroupedMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly SizeMessage2GroupedMessage defaultInstance = new SizeMessage2GroupedMessage().MakeReadOnly();
     public static SizeMessage2GroupedMessage DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override SizeMessage2GroupedMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SizeMessage2GroupedMessage ThisMessage {
@@ -4263,6 +4279,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SizeMessage2GroupedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SizeMessage2GroupedMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -4278,22 +4298,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SizeMessage2GroupedMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SizeMessage2GroupedMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SizeMessage2GroupedMessage result;
+      private bool resultIsReadOnly;
+      private SizeMessage2GroupedMessage result;
       
       private SizeMessage2GroupedMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SizeMessage2GroupedMessage original = result;
           result = new SizeMessage2GroupedMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -4308,13 +4328,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SizeMessage2GroupedMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -4330,11 +4350,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SizeMessage2GroupedMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       

src/ProtocolBuffers.Test/TestProtos/UnitTestGoogleSpeedProtoFile.cs

@@ -137,7 +137,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage1 : pb::GeneratedMessage<SpeedMessage1, SpeedMessage1.Builder> {
-    private static readonly SpeedMessage1 defaultInstance = new Builder().BuildPartial();
+    private static readonly SpeedMessage1 defaultInstance = new SpeedMessage1().MakeReadOnly();
     private static readonly string[] _speedMessage1FieldNames = new string[] { "field1", "field100", "field101", "field102", "field103", "field104", "field12", "field128", "field129", "field13", "field130", "field131", "field14", "field15", "field150", "field16", "field17", "field18", "field2", "field22", "field23", "field24", "field25", "field271", "field272", "field280", "field29", "field3", "field30", "field4", "field5", "field59", "field6", "field60", "field67", "field68", "field7", "field78", "field80", "field81", "field9" };
     private static readonly uint[] _speedMessage1FieldTags = new uint[] { 10, 800, 808, 818, 826, 832, 96, 1024, 1034, 104, 1040, 1048, 112, 122, 1200, 128, 136, 146, 16, 176, 184, 192, 200, 2168, 2176, 2240, 232, 24, 240, 34, 41, 472, 48, 480, 536, 544, 58, 624, 640, 648, 74 };
     public static SpeedMessage1 DefaultInstance {
@@ -145,7 +145,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage1 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage1 ThisMessage {
@@ -506,12 +506,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int Field15FieldNumber = 15;
     private bool hasField15;
-    private global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage field15_;
     public bool HasField15 {
       get { return hasField15; }
     }
     public global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage Field15 {
-      get { return field15_; }
+      get { return field15_ ?? global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance; }
     }
     
     public const int Field78FieldNumber = 78;
@@ -881,6 +881,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SpeedMessage1 MakeReadOnly() {
+      field5_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -896,22 +901,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SpeedMessage1();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SpeedMessage1 cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SpeedMessage1 result;
+      private bool resultIsReadOnly;
+      private SpeedMessage1 result;
       
       private SpeedMessage1 PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SpeedMessage1 original = result;
           result = new SpeedMessage1();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -926,13 +931,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SpeedMessage1();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -948,12 +953,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SpeedMessage1 BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.field5_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -2052,7 +2056,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearField15() {
         PrepareBuilder();
         result.hasField15 = false;
-        result.field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance;
+        result.field15_ = null;
         return this;
       }
       
@@ -2186,7 +2190,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage1SubMessage : pb::GeneratedMessage<SpeedMessage1SubMessage, SpeedMessage1SubMessage.Builder> {
-    private static readonly SpeedMessage1SubMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly SpeedMessage1SubMessage defaultInstance = new SpeedMessage1SubMessage().MakeReadOnly();
     private static readonly string[] _speedMessage1SubMessageFieldNames = new string[] { "field1", "field12", "field13", "field14", "field15", "field16", "field19", "field2", "field20", "field203", "field204", "field205", "field206", "field207", "field21", "field22", "field23", "field28", "field3", "field300" };
     private static readonly uint[] _speedMessage1SubMessageFieldTags = new uint[] { 8, 96, 104, 112, 122, 128, 152, 16, 160, 1629, 1632, 1642, 1648, 1656, 169, 176, 184, 224, 24, 2400 };
     public static SpeedMessage1SubMessage DefaultInstance {
@@ -2194,7 +2198,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage1SubMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage1SubMessage ThisMessage {
@@ -2588,6 +2592,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage1SubMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SpeedMessage1SubMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -2603,22 +2611,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SpeedMessage1SubMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SpeedMessage1SubMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SpeedMessage1SubMessage result;
+      private bool resultIsReadOnly;
+      private SpeedMessage1SubMessage result;
       
       private SpeedMessage1SubMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SpeedMessage1SubMessage original = result;
           result = new SpeedMessage1SubMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -2633,13 +2641,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SpeedMessage1SubMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -2655,11 +2663,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SpeedMessage1SubMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -3286,7 +3294,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage2 : pb::GeneratedMessage<SpeedMessage2, SpeedMessage2.Builder> {
-    private static readonly SpeedMessage2 defaultInstance = new Builder().BuildPartial();
+    private static readonly SpeedMessage2 defaultInstance = new SpeedMessage2().MakeReadOnly();
     private static readonly string[] _speedMessage2FieldNames = new string[] { "field1", "field109", "field127", "field128", "field129", "field130", "field131", "field2", "field205", "field206", "field21", "field210", "field211", "field212", "field213", "field216", "field217", "field218", "field220", "field221", "field222", "field25", "field3", "field30", "field4", "field6", "field63", "field71", "field75", "group1" };
     private static readonly uint[] _speedMessage2FieldTags = new uint[] { 10, 872, 1018, 1026, 1032, 1040, 1048, 18, 1640, 1648, 168, 1680, 1688, 1696, 1704, 1728, 1736, 1744, 1760, 1768, 1781, 205, 24, 240, 32, 50, 504, 568, 600, 83 };
     public static SpeedMessage2 DefaultInstance {
@@ -3294,7 +3302,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage2 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage2 ThisMessage {
@@ -3318,7 +3326,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Group1 : pb::GeneratedMessage<Group1, Group1.Builder> {
-        private static readonly Group1 defaultInstance = new Builder().BuildPartial();
+        private static readonly Group1 defaultInstance = new Group1().MakeReadOnly();
         private static readonly string[] _group1FieldNames = new string[] { "field11", "field12", "field13", "field14", "field15", "field16", "field20", "field22", "field24", "field26", "field27", "field28", "field29", "field31", "field5", "field73" };
         private static readonly uint[] _group1FieldTags = new uint[] { 93, 98, 106, 114, 120, 130, 160, 178, 194, 213, 218, 224, 234, 250, 40, 584 };
         public static Group1 DefaultInstance {
@@ -3326,7 +3334,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Group1 DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Group1 ThisMessage {
@@ -3500,12 +3508,12 @@ namespace Google.ProtocolBuffers.TestProtos {
         
         public const int Field31FieldNumber = 31;
         private bool hasField31;
-        private global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance;
+        private global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage field31_;
         public bool HasField31 {
           get { return hasField31; }
         }
         public global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage Field31 {
-          get { return field31_; }
+          get { return field31_ ?? global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance; }
         }
         
         public override bool IsInitialized {
@@ -3676,6 +3684,13 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Group1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private Group1 MakeReadOnly() {
+          field14_.MakeReadOnly();
+          field22_.MakeReadOnly();
+          field73_.MakeReadOnly();
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -3691,22 +3706,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new Group1();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(Group1 cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          Group1 result;
+          private bool resultIsReadOnly;
+          private Group1 result;
           
           private Group1 PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               Group1 original = result;
               result = new Group1();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -3721,13 +3736,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new Group1();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -3743,14 +3758,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Group1 BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            result.field14_.MakeReadOnly();
-            result.field22_.MakeReadOnly();
-            result.field73_.MakeReadOnly();
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessage other) {
@@ -4314,7 +4326,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           public Builder ClearField31() {
             PrepareBuilder();
             result.hasField31 = false;
-            result.field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance;
+            result.field31_ = null;
             return this;
           }
         }
@@ -4884,6 +4896,14 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SpeedMessage2 MakeReadOnly() {
+      group1_.MakeReadOnly();
+      field128_.MakeReadOnly();
+      field127_.MakeReadOnly();
+      field130_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -4899,22 +4919,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SpeedMessage2();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SpeedMessage2 cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SpeedMessage2 result;
+      private bool resultIsReadOnly;
+      private SpeedMessage2 result;
       
       private SpeedMessage2 PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SpeedMessage2 original = result;
           result = new SpeedMessage2();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -4929,13 +4949,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SpeedMessage2();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -4951,15 +4971,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SpeedMessage2 BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.group1_.MakeReadOnly();
-        result.field128_.MakeReadOnly();
-        result.field127_.MakeReadOnly();
-        result.field130_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -5908,7 +5924,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage2GroupedMessage : pb::GeneratedMessage<SpeedMessage2GroupedMessage, SpeedMessage2GroupedMessage.Builder> {
-    private static readonly SpeedMessage2GroupedMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly SpeedMessage2GroupedMessage defaultInstance = new SpeedMessage2GroupedMessage().MakeReadOnly();
     private static readonly string[] _speedMessage2GroupedMessageFieldNames = new string[] { "field1", "field10", "field11", "field2", "field3", "field4", "field5", "field6", "field7", "field8", "field9" };
     private static readonly uint[] _speedMessage2GroupedMessageFieldTags = new uint[] { 13, 85, 88, 21, 29, 32, 40, 48, 56, 69, 72 };
     public static SpeedMessage2GroupedMessage DefaultInstance {
@@ -5916,7 +5932,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage2GroupedMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage2GroupedMessage ThisMessage {
@@ -6162,6 +6178,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SpeedMessage2GroupedMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SpeedMessage2GroupedMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -6177,22 +6197,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SpeedMessage2GroupedMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SpeedMessage2GroupedMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SpeedMessage2GroupedMessage result;
+      private bool resultIsReadOnly;
+      private SpeedMessage2GroupedMessage result;
       
       private SpeedMessage2GroupedMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SpeedMessage2GroupedMessage original = result;
           result = new SpeedMessage2GroupedMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -6207,13 +6227,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SpeedMessage2GroupedMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -6229,11 +6249,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SpeedMessage2GroupedMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestImportLiteProtoFile.cs

@@ -43,7 +43,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessageLite : pb::GeneratedMessageLite<ImportMessageLite, ImportMessageLite.Builder> {
-    private static readonly ImportMessageLite defaultInstance = new Builder().BuildPartial();
+    private static readonly ImportMessageLite defaultInstance = new ImportMessageLite().MakeReadOnly();
     private static readonly string[] _importMessageLiteFieldNames = new string[] { "d" };
     private static readonly uint[] _importMessageLiteFieldTags = new uint[] { 8 };
     public static ImportMessageLite DefaultInstance {
@@ -51,7 +51,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessageLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessageLite ThisMessage {
@@ -146,6 +146,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessageLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private ImportMessageLite MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -161,22 +165,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new ImportMessageLite();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(ImportMessageLite cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      ImportMessageLite result;
+      private bool resultIsReadOnly;
+      private ImportMessageLite result;
       
       private ImportMessageLite PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           ImportMessageLite original = result;
           result = new ImportMessageLite();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -191,13 +195,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new ImportMessageLite();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -209,11 +213,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override ImportMessageLite BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessageLite other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs

@@ -71,7 +71,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessage : pb::GeneratedMessage<ImportMessage, ImportMessage.Builder> {
-    private static readonly ImportMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly ImportMessage defaultInstance = new ImportMessage().MakeReadOnly();
     private static readonly string[] _importMessageFieldNames = new string[] { "d" };
     private static readonly uint[] _importMessageFieldTags = new uint[] { 8 };
     public static ImportMessage DefaultInstance {
@@ -79,7 +79,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessage ThisMessage {
@@ -165,6 +165,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private ImportMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -180,22 +184,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new ImportMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(ImportMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      ImportMessage result;
+      private bool resultIsReadOnly;
+      private ImportMessage result;
       
       private ImportMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           ImportMessage original = result;
           result = new ImportMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -210,13 +214,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new ImportMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -232,11 +236,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override ImportMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs

@@ -103,7 +103,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSet : pb::ExtendableMessage<TestMessageSet, TestMessageSet.Builder> {
-    private static readonly TestMessageSet defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMessageSet defaultInstance = new TestMessageSet().MakeReadOnly();
     private static readonly string[] _testMessageSetFieldNames = new string[] {  };
     private static readonly uint[] _testMessageSetFieldTags = new uint[] {  };
     public static TestMessageSet DefaultInstance {
@@ -111,7 +111,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSet DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSet ThisMessage {
@@ -185,6 +185,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSet ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMessageSet MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -200,22 +204,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMessageSet();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMessageSet cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMessageSet result;
+      private bool resultIsReadOnly;
+      private TestMessageSet result;
       
       private TestMessageSet PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMessageSet original = result;
           result = new TestMessageSet();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -230,13 +234,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMessageSet();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -252,11 +256,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMessageSet BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -334,7 +338,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSetContainer : pb::GeneratedMessage<TestMessageSetContainer, TestMessageSetContainer.Builder> {
-    private static readonly TestMessageSetContainer defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMessageSetContainer defaultInstance = new TestMessageSetContainer().MakeReadOnly();
     private static readonly string[] _testMessageSetContainerFieldNames = new string[] { "message_set" };
     private static readonly uint[] _testMessageSetContainerFieldTags = new uint[] { 10 };
     public static TestMessageSetContainer DefaultInstance {
@@ -342,7 +346,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSetContainer DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSetContainer ThisMessage {
@@ -359,12 +363,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int MessageSetFieldNumber = 1;
     private bool hasMessageSet;
-    private global::Google.ProtocolBuffers.TestProtos.TestMessageSet messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestMessageSet messageSet_;
     public bool HasMessageSet {
       get { return hasMessageSet; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestMessageSet MessageSet {
-      get { return messageSet_; }
+      get { return messageSet_ ?? global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance; }
     }
     
     public override bool IsInitialized {
@@ -428,6 +432,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSetContainer ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMessageSetContainer MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -443,22 +451,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMessageSetContainer();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMessageSetContainer cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMessageSetContainer result;
+      private bool resultIsReadOnly;
+      private TestMessageSetContainer result;
       
       private TestMessageSetContainer PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMessageSetContainer original = result;
           result = new TestMessageSetContainer();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -473,13 +481,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMessageSetContainer();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -495,11 +503,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMessageSetContainer BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -615,7 +623,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageSet() {
         PrepareBuilder();
         result.hasMessageSet = false;
-        result.messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance;
+        result.messageSet_ = null;
         return this;
       }
     }
@@ -628,7 +636,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSetExtension1 : pb::GeneratedMessage<TestMessageSetExtension1, TestMessageSetExtension1.Builder> {
-    private static readonly TestMessageSetExtension1 defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMessageSetExtension1 defaultInstance = new TestMessageSetExtension1().MakeReadOnly();
     private static readonly string[] _testMessageSetExtension1FieldNames = new string[] { "i" };
     private static readonly uint[] _testMessageSetExtension1FieldTags = new uint[] { 120 };
     public static TestMessageSetExtension1 DefaultInstance {
@@ -636,7 +644,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSetExtension1 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSetExtension1 ThisMessage {
@@ -724,6 +732,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSetExtension1 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMessageSetExtension1 MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -739,22 +751,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMessageSetExtension1();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMessageSetExtension1 cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMessageSetExtension1 result;
+      private bool resultIsReadOnly;
+      private TestMessageSetExtension1 result;
       
       private TestMessageSetExtension1 PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMessageSetExtension1 original = result;
           result = new TestMessageSetExtension1();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -769,13 +781,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMessageSetExtension1();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -791,11 +803,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMessageSetExtension1 BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -899,7 +911,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSetExtension2 : pb::GeneratedMessage<TestMessageSetExtension2, TestMessageSetExtension2.Builder> {
-    private static readonly TestMessageSetExtension2 defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMessageSetExtension2 defaultInstance = new TestMessageSetExtension2().MakeReadOnly();
     private static readonly string[] _testMessageSetExtension2FieldNames = new string[] { "str" };
     private static readonly uint[] _testMessageSetExtension2FieldTags = new uint[] { 202 };
     public static TestMessageSetExtension2 DefaultInstance {
@@ -907,7 +919,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSetExtension2 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSetExtension2 ThisMessage {
@@ -995,6 +1007,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMessageSetExtension2 ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMessageSetExtension2 MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1010,22 +1026,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMessageSetExtension2();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMessageSetExtension2 cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMessageSetExtension2 result;
+      private bool resultIsReadOnly;
+      private TestMessageSetExtension2 result;
       
       private TestMessageSetExtension2 PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMessageSetExtension2 original = result;
           result = new TestMessageSetExtension2();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1040,13 +1056,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMessageSetExtension2();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1062,11 +1078,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMessageSetExtension2 BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -1171,7 +1187,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class RawMessageSet : pb::GeneratedMessage<RawMessageSet, RawMessageSet.Builder> {
-    private static readonly RawMessageSet defaultInstance = new Builder().BuildPartial();
+    private static readonly RawMessageSet defaultInstance = new RawMessageSet().MakeReadOnly();
     private static readonly string[] _rawMessageSetFieldNames = new string[] { "item" };
     private static readonly uint[] _rawMessageSetFieldTags = new uint[] { 11 };
     public static RawMessageSet DefaultInstance {
@@ -1179,7 +1195,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override RawMessageSet DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override RawMessageSet ThisMessage {
@@ -1203,7 +1219,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Item : pb::GeneratedMessage<Item, Item.Builder> {
-        private static readonly Item defaultInstance = new Builder().BuildPartial();
+        private static readonly Item defaultInstance = new Item().MakeReadOnly();
         private static readonly string[] _itemFieldNames = new string[] { "message", "type_id" };
         private static readonly uint[] _itemFieldTags = new uint[] { 26, 16 };
         public static Item DefaultInstance {
@@ -1211,7 +1227,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Item DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Item ThisMessage {
@@ -1315,6 +1331,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Item ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private Item MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1330,22 +1350,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new Item();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(Item cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          Item result;
+          private bool resultIsReadOnly;
+          private Item result;
           
           private Item PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               Item original = result;
               result = new Item();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -1360,13 +1380,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new Item();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -1382,11 +1402,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Item BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessage other) {
@@ -1590,6 +1610,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RawMessageSet ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private RawMessageSet MakeReadOnly() {
+      item_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1605,22 +1630,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new RawMessageSet();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(RawMessageSet cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      RawMessageSet result;
+      private bool resultIsReadOnly;
+      private RawMessageSet result;
       
       private RawMessageSet PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           RawMessageSet original = result;
           result = new RawMessageSet();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1635,13 +1660,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new RawMessageSet();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1657,12 +1682,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override RawMessageSet BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.item_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestNoGenericServicesProtoFile.cs

@@ -81,7 +81,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessage : pb::ExtendableMessage<TestMessage, TestMessage.Builder> {
-    private static readonly TestMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMessage defaultInstance = new TestMessage().MakeReadOnly();
     private static readonly string[] _testMessageFieldNames = new string[] { "a" };
     private static readonly uint[] _testMessageFieldTags = new uint[] { 8 };
     public static TestMessage DefaultInstance {
@@ -89,7 +89,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
     }
     
     public override TestMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessage ThisMessage {
@@ -179,6 +179,10 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
     public static TestMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -194,22 +198,22 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMessage result;
+      private bool resultIsReadOnly;
+      private TestMessage result;
       
       private TestMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMessage original = result;
           result = new TestMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -224,13 +228,13 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -246,11 +250,11 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
       }
       
       public override TestMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers.Test/TestProtos/UnitTestOptimizeForProtoFile.cs

@@ -85,13 +85,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestOptimizedForSize : pb::ExtendableMessage<TestOptimizedForSize, TestOptimizedForSize.Builder> {
-    private static readonly TestOptimizedForSize defaultInstance = new Builder().BuildPartial();
+    private static readonly TestOptimizedForSize defaultInstance = new TestOptimizedForSize().MakeReadOnly();
     public static TestOptimizedForSize DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestOptimizedForSize DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestOptimizedForSize ThisMessage {
@@ -122,12 +122,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int MsgFieldNumber = 19;
     private bool hasMsg;
-    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.ForeignMessage msg_;
     public bool HasMsg {
       get { return hasMsg; }
     }
     public global::Google.ProtocolBuffers.TestProtos.ForeignMessage Msg {
-      get { return msg_; }
+      get { return msg_ ?? global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance; }
     }
     
     public static TestOptimizedForSize ParseFrom(pb::ByteString data) {
@@ -160,6 +160,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestOptimizedForSize MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -175,22 +179,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestOptimizedForSize();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestOptimizedForSize cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestOptimizedForSize result;
+      private bool resultIsReadOnly;
+      private TestOptimizedForSize result;
       
       private TestOptimizedForSize PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestOptimizedForSize original = result;
           result = new TestOptimizedForSize();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -205,13 +209,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestOptimizedForSize();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -227,11 +231,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -291,7 +295,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMsg() {
         PrepareBuilder();
         result.hasMsg = false;
-        result.msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;
+        result.msg_ = null;
         return this;
       }
     }
@@ -304,13 +308,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestRequiredOptimizedForSize : pb::GeneratedMessage<TestRequiredOptimizedForSize, TestRequiredOptimizedForSize.Builder> {
-    private static readonly TestRequiredOptimizedForSize defaultInstance = new Builder().BuildPartial();
+    private static readonly TestRequiredOptimizedForSize defaultInstance = new TestRequiredOptimizedForSize().MakeReadOnly();
     public static TestRequiredOptimizedForSize DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestRequiredOptimizedForSize DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestRequiredOptimizedForSize ThisMessage {
@@ -365,6 +369,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequiredOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestRequiredOptimizedForSize MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -380,22 +388,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestRequiredOptimizedForSize();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestRequiredOptimizedForSize cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestRequiredOptimizedForSize result;
+      private bool resultIsReadOnly;
+      private TestRequiredOptimizedForSize result;
       
       private TestRequiredOptimizedForSize PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestRequiredOptimizedForSize original = result;
           result = new TestRequiredOptimizedForSize();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -410,13 +418,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestRequiredOptimizedForSize();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -432,11 +440,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestRequiredOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -469,13 +477,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestOptionalOptimizedForSize : pb::GeneratedMessage<TestOptionalOptimizedForSize, TestOptionalOptimizedForSize.Builder> {
-    private static readonly TestOptionalOptimizedForSize defaultInstance = new Builder().BuildPartial();
+    private static readonly TestOptionalOptimizedForSize defaultInstance = new TestOptionalOptimizedForSize().MakeReadOnly();
     public static TestOptionalOptimizedForSize DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestOptionalOptimizedForSize DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestOptionalOptimizedForSize ThisMessage {
@@ -492,12 +500,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int OFieldNumber = 1;
     private bool hasO;
-    private global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize o_ = global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize o_;
     public bool HasO {
       get { return hasO; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize O {
-      get { return o_; }
+      get { return o_ ?? global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance; }
     }
     
     public static TestOptionalOptimizedForSize ParseFrom(pb::ByteString data) {
@@ -530,6 +538,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestOptionalOptimizedForSize ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestOptionalOptimizedForSize MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -545,22 +557,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestOptionalOptimizedForSize();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestOptionalOptimizedForSize cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestOptionalOptimizedForSize result;
+      private bool resultIsReadOnly;
+      private TestOptionalOptimizedForSize result;
       
       private TestOptionalOptimizedForSize PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestOptionalOptimizedForSize original = result;
           result = new TestOptionalOptimizedForSize();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -575,13 +587,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestOptionalOptimizedForSize();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -597,11 +609,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestOptionalOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -641,7 +653,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearO() {
         PrepareBuilder();
         result.hasO = false;
-        result.o_ = global::Google.ProtocolBuffers.TestProtos.TestRequiredOptimizedForSize.DefaultInstance;
+        result.o_ = null;
         return this;
       }
     }

src/ProtocolBuffers.Test/TestProtos/UnitTestRpcInterop.cs

@@ -83,7 +83,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SearchRequest : pb::GeneratedMessage<SearchRequest, SearchRequest.Builder> {
-    private static readonly SearchRequest defaultInstance = new Builder().BuildPartial();
+    private static readonly SearchRequest defaultInstance = new SearchRequest().MakeReadOnly();
     private static readonly string[] _searchRequestFieldNames = new string[] { "Criteria" };
     private static readonly uint[] _searchRequestFieldTags = new uint[] { 10 };
     public static SearchRequest DefaultInstance {
@@ -91,7 +91,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SearchRequest DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SearchRequest ThisMessage {
@@ -184,6 +184,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SearchRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SearchRequest MakeReadOnly() {
+      criteria_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -199,22 +204,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SearchRequest();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SearchRequest cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SearchRequest result;
+      private bool resultIsReadOnly;
+      private SearchRequest result;
       
       private SearchRequest PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SearchRequest original = result;
           result = new SearchRequest();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -229,13 +234,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SearchRequest();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -251,12 +256,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SearchRequest BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.criteria_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -372,7 +376,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SearchResponse : pb::GeneratedMessage<SearchResponse, SearchResponse.Builder> {
-    private static readonly SearchResponse defaultInstance = new Builder().BuildPartial();
+    private static readonly SearchResponse defaultInstance = new SearchResponse().MakeReadOnly();
     private static readonly string[] _searchResponseFieldNames = new string[] { "results" };
     private static readonly uint[] _searchResponseFieldTags = new uint[] { 10 };
     public static SearchResponse DefaultInstance {
@@ -380,7 +384,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SearchResponse DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SearchResponse ThisMessage {
@@ -404,7 +408,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class ResultItem : pb::GeneratedMessage<ResultItem, ResultItem.Builder> {
-        private static readonly ResultItem defaultInstance = new Builder().BuildPartial();
+        private static readonly ResultItem defaultInstance = new ResultItem().MakeReadOnly();
         private static readonly string[] _resultItemFieldNames = new string[] { "name", "url" };
         private static readonly uint[] _resultItemFieldTags = new uint[] { 18, 10 };
         public static ResultItem DefaultInstance {
@@ -412,7 +416,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override ResultItem DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override ResultItem ThisMessage {
@@ -515,6 +519,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static ResultItem ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private ResultItem MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -530,22 +538,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new ResultItem();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(ResultItem cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          ResultItem result;
+          private bool resultIsReadOnly;
+          private ResultItem result;
           
           private ResultItem PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               ResultItem original = result;
               result = new ResultItem();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -560,13 +568,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new ResultItem();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -582,11 +590,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override ResultItem BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessage other) {
@@ -794,6 +802,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static SearchResponse ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private SearchResponse MakeReadOnly() {
+      results_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -809,22 +822,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new SearchResponse();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(SearchResponse cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      SearchResponse result;
+      private bool resultIsReadOnly;
+      private SearchResponse result;
       
       private SearchResponse PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           SearchResponse original = result;
           result = new SearchResponse();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -839,13 +852,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new SearchResponse();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -861,12 +874,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SearchResponse BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.results_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -994,7 +1006,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class RefineSearchRequest : pb::GeneratedMessage<RefineSearchRequest, RefineSearchRequest.Builder> {
-    private static readonly RefineSearchRequest defaultInstance = new Builder().BuildPartial();
+    private static readonly RefineSearchRequest defaultInstance = new RefineSearchRequest().MakeReadOnly();
     private static readonly string[] _refineSearchRequestFieldNames = new string[] { "Criteria", "previous_results" };
     private static readonly uint[] _refineSearchRequestFieldTags = new uint[] { 10, 18 };
     public static RefineSearchRequest DefaultInstance {
@@ -1002,7 +1014,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override RefineSearchRequest DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override RefineSearchRequest ThisMessage {
@@ -1031,12 +1043,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int PreviousResultsFieldNumber = 2;
     private bool hasPreviousResults;
-    private global::Google.ProtocolBuffers.TestProtos.SearchResponse previousResults_ = global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.SearchResponse previousResults_;
     public bool HasPreviousResults {
       get { return hasPreviousResults; }
     }
     public global::Google.ProtocolBuffers.TestProtos.SearchResponse PreviousResults {
-      get { return previousResults_; }
+      get { return previousResults_ ?? global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance; }
     }
     
     public override bool IsInitialized {
@@ -1113,6 +1125,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static RefineSearchRequest ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private RefineSearchRequest MakeReadOnly() {
+      criteria_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1128,22 +1145,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new RefineSearchRequest();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(RefineSearchRequest cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      RefineSearchRequest result;
+      private bool resultIsReadOnly;
+      private RefineSearchRequest result;
       
       private RefineSearchRequest PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           RefineSearchRequest original = result;
           result = new RefineSearchRequest();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1158,13 +1175,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new RefineSearchRequest();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1180,12 +1197,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override RefineSearchRequest BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.criteria_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -1340,7 +1356,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearPreviousResults() {
         PrepareBuilder();
         result.hasPreviousResults = false;
-        result.previousResults_ = global::Google.ProtocolBuffers.TestProtos.SearchResponse.DefaultInstance;
+        result.previousResults_ = null;
         return this;
       }
     }

src/ProtocolBuffers.Test/TestProtos/UnitTestXmlSerializerTestProtoFile.cs

@@ -138,7 +138,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlChild : pb::GeneratedMessage<TestXmlChild, TestXmlChild.Builder> {
-    private static readonly TestXmlChild defaultInstance = new Builder().BuildPartial();
+    private static readonly TestXmlChild defaultInstance = new TestXmlChild().MakeReadOnly();
     private static readonly string[] _testXmlChildFieldNames = new string[] { "binary", "options" };
     private static readonly uint[] _testXmlChildFieldTags = new uint[] { 34, 24 };
     public static TestXmlChild DefaultInstance {
@@ -146,7 +146,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlChild DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlChild ThisMessage {
@@ -257,6 +257,11 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlChild ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestXmlChild MakeReadOnly() {
+      options_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -272,22 +277,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestXmlChild();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestXmlChild cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestXmlChild result;
+      private bool resultIsReadOnly;
+      private TestXmlChild result;
       
       private TestXmlChild PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestXmlChild original = result;
           result = new TestXmlChild();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -302,13 +307,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestXmlChild();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -324,12 +329,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestXmlChild BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.options_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -481,7 +485,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlNoFields : pb::GeneratedMessage<TestXmlNoFields, TestXmlNoFields.Builder> {
-    private static readonly TestXmlNoFields defaultInstance = new Builder().BuildPartial();
+    private static readonly TestXmlNoFields defaultInstance = new TestXmlNoFields().MakeReadOnly();
     private static readonly string[] _testXmlNoFieldsFieldNames = new string[] {  };
     private static readonly uint[] _testXmlNoFieldsFieldTags = new uint[] {  };
     public static TestXmlNoFields DefaultInstance {
@@ -489,7 +493,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlNoFields DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlNoFields ThisMessage {
@@ -559,6 +563,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlNoFields ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestXmlNoFields MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -574,22 +582,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestXmlNoFields();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestXmlNoFields cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestXmlNoFields result;
+      private bool resultIsReadOnly;
+      private TestXmlNoFields result;
       
       private TestXmlNoFields PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestXmlNoFields original = result;
           result = new TestXmlNoFields();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -604,13 +612,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestXmlNoFields();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -626,11 +634,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestXmlNoFields BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -707,7 +715,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlRescursive : pb::GeneratedMessage<TestXmlRescursive, TestXmlRescursive.Builder> {
-    private static readonly TestXmlRescursive defaultInstance = new Builder().BuildPartial();
+    private static readonly TestXmlRescursive defaultInstance = new TestXmlRescursive().MakeReadOnly();
     private static readonly string[] _testXmlRescursiveFieldNames = new string[] { "child" };
     private static readonly uint[] _testXmlRescursiveFieldTags = new uint[] { 10 };
     public static TestXmlRescursive DefaultInstance {
@@ -715,7 +723,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlRescursive DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlRescursive ThisMessage {
@@ -732,12 +740,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int ChildFieldNumber = 1;
     private bool hasChild;
-    private global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive child_;
     public bool HasChild {
       get { return hasChild; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive Child {
-      get { return child_; }
+      get { return child_ ?? global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance; }
     }
     
     public override bool IsInitialized {
@@ -801,6 +809,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlRescursive ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestXmlRescursive MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -816,22 +828,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestXmlRescursive();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestXmlRescursive cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestXmlRescursive result;
+      private bool resultIsReadOnly;
+      private TestXmlRescursive result;
       
       private TestXmlRescursive PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestXmlRescursive original = result;
           result = new TestXmlRescursive();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -846,13 +858,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestXmlRescursive();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -868,11 +880,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestXmlRescursive BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -988,7 +1000,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearChild() {
         PrepareBuilder();
         result.hasChild = false;
-        result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance;
+        result.child_ = null;
         return this;
       }
     }
@@ -1001,7 +1013,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlMessage : pb::ExtendableMessage<TestXmlMessage, TestXmlMessage.Builder> {
-    private static readonly TestXmlMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly TestXmlMessage defaultInstance = new TestXmlMessage().MakeReadOnly();
     private static readonly string[] _testXmlMessageFieldNames = new string[] { "child", "children", "number", "numbers", "text", "textlines", "valid" };
     private static readonly uint[] _testXmlMessageFieldTags = new uint[] { 10, 3211, 48, 16, 26, 5602, 40 };
     public static TestXmlMessage DefaultInstance {
@@ -1009,7 +1021,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlMessage ThisMessage {
@@ -1033,7 +1045,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Children : pb::GeneratedMessage<Children, Children.Builder> {
-        private static readonly Children defaultInstance = new Builder().BuildPartial();
+        private static readonly Children defaultInstance = new Children().MakeReadOnly();
         private static readonly string[] _childrenFieldNames = new string[] { "binary", "options" };
         private static readonly uint[] _childrenFieldTags = new uint[] { 34, 24 };
         public static Children DefaultInstance {
@@ -1041,7 +1053,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Children DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Children ThisMessage {
@@ -1152,6 +1164,11 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Children ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private Children MakeReadOnly() {
+          options_.MakeReadOnly();
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1167,22 +1184,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new Children();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(Children cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          Children result;
+          private bool resultIsReadOnly;
+          private Children result;
           
           private Children PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               Children original = result;
               result = new Children();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -1197,13 +1214,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new Children();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -1219,12 +1236,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Children BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            result.options_.MakeReadOnly();
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessage other) {
@@ -1431,12 +1447,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int ChildFieldNumber = 1;
     private bool hasChild;
-    private global::Google.ProtocolBuffers.TestProtos.TestXmlChild child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestXmlChild child_;
     public bool HasChild {
       get { return hasChild; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestXmlChild Child {
-      get { return child_; }
+      get { return child_ ?? global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance; }
     }
     
     public const int ChildrenFieldNumber = 401;
@@ -1562,6 +1578,13 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestXmlMessage MakeReadOnly() {
+      numbers_.MakeReadOnly();
+      textlines_.MakeReadOnly();
+      children_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1577,22 +1600,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestXmlMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestXmlMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestXmlMessage result;
+      private bool resultIsReadOnly;
+      private TestXmlMessage result;
       
       private TestXmlMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestXmlMessage original = result;
           result = new TestXmlMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1607,13 +1630,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestXmlMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1629,14 +1652,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestXmlMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.numbers_.MakeReadOnly();
-        result.textlines_.MakeReadOnly();
-        result.children_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -1919,7 +1939,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearChild() {
         PrepareBuilder();
         result.hasChild = false;
-        result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance;
+        result.child_ = null;
         return this;
       }
       
@@ -1976,7 +1996,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlExtension : pb::GeneratedMessage<TestXmlExtension, TestXmlExtension.Builder> {
-    private static readonly TestXmlExtension defaultInstance = new Builder().BuildPartial();
+    private static readonly TestXmlExtension defaultInstance = new TestXmlExtension().MakeReadOnly();
     private static readonly string[] _testXmlExtensionFieldNames = new string[] { "number" };
     private static readonly uint[] _testXmlExtensionFieldTags = new uint[] { 8 };
     public static TestXmlExtension DefaultInstance {
@@ -1984,7 +2004,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlExtension DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlExtension ThisMessage {
@@ -2071,6 +2091,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestXmlExtension ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestXmlExtension MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -2086,22 +2110,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestXmlExtension();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestXmlExtension cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestXmlExtension result;
+      private bool resultIsReadOnly;
+      private TestXmlExtension result;
       
       private TestXmlExtension PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestXmlExtension original = result;
           result = new TestXmlExtension();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -2116,13 +2140,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestXmlExtension();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -2138,11 +2162,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestXmlExtension BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffers/DescriptorProtos/CSharpOptions.cs

@@ -125,7 +125,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpFileOptions : pb::GeneratedMessage<CSharpFileOptions, CSharpFileOptions.Builder> {
-    private static readonly CSharpFileOptions defaultInstance = new Builder().BuildPartial();
+    private static readonly CSharpFileOptions defaultInstance = new CSharpFileOptions().MakeReadOnly();
     private static readonly string[] _cSharpFileOptionsFieldNames = new string[] { "cls_compliance", "code_contracts", "expand_namespace_directories", "file_extension", "ignore_google_protobuf", "multiple_files", "namespace", "nest_classes", "output_directory", "public_classes", "service_generator_type", "umbrella_classname", "umbrella_namespace" };
     private static readonly uint[] _cSharpFileOptionsFieldTags = new uint[] { 64, 48, 56, 1770, 1792, 32, 10, 40, 1786, 24, 1800, 18, 1778 };
     public static CSharpFileOptions DefaultInstance {
@@ -133,7 +133,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpFileOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpFileOptions ThisMessage {
@@ -411,6 +411,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpFileOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private CSharpFileOptions MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -426,22 +430,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new CSharpFileOptions();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(CSharpFileOptions cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      CSharpFileOptions result;
+      private bool resultIsReadOnly;
+      private CSharpFileOptions result;
       
       private CSharpFileOptions PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           CSharpFileOptions original = result;
           result = new CSharpFileOptions();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -456,13 +460,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new CSharpFileOptions();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -478,11 +482,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override CSharpFileOptions BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -923,7 +927,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpFieldOptions : pb::GeneratedMessage<CSharpFieldOptions, CSharpFieldOptions.Builder> {
-    private static readonly CSharpFieldOptions defaultInstance = new Builder().BuildPartial();
+    private static readonly CSharpFieldOptions defaultInstance = new CSharpFieldOptions().MakeReadOnly();
     private static readonly string[] _cSharpFieldOptionsFieldNames = new string[] { "property_name" };
     private static readonly uint[] _cSharpFieldOptionsFieldTags = new uint[] { 10 };
     public static CSharpFieldOptions DefaultInstance {
@@ -931,7 +935,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpFieldOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpFieldOptions ThisMessage {
@@ -1017,6 +1021,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpFieldOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private CSharpFieldOptions MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1032,22 +1040,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new CSharpFieldOptions();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(CSharpFieldOptions cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      CSharpFieldOptions result;
+      private bool resultIsReadOnly;
+      private CSharpFieldOptions result;
       
       private CSharpFieldOptions PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           CSharpFieldOptions original = result;
           result = new CSharpFieldOptions();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1062,13 +1070,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new CSharpFieldOptions();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1084,11 +1092,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override CSharpFieldOptions BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -1193,7 +1201,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpServiceOptions : pb::GeneratedMessage<CSharpServiceOptions, CSharpServiceOptions.Builder> {
-    private static readonly CSharpServiceOptions defaultInstance = new Builder().BuildPartial();
+    private static readonly CSharpServiceOptions defaultInstance = new CSharpServiceOptions().MakeReadOnly();
     private static readonly string[] _cSharpServiceOptionsFieldNames = new string[] { "interface_id" };
     private static readonly uint[] _cSharpServiceOptionsFieldTags = new uint[] { 10 };
     public static CSharpServiceOptions DefaultInstance {
@@ -1201,7 +1209,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpServiceOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpServiceOptions ThisMessage {
@@ -1287,6 +1295,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpServiceOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private CSharpServiceOptions MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1302,22 +1314,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new CSharpServiceOptions();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(CSharpServiceOptions cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      CSharpServiceOptions result;
+      private bool resultIsReadOnly;
+      private CSharpServiceOptions result;
       
       private CSharpServiceOptions PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           CSharpServiceOptions original = result;
           result = new CSharpServiceOptions();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1332,13 +1344,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new CSharpServiceOptions();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1354,11 +1366,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override CSharpServiceOptions BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {
@@ -1463,7 +1475,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpMethodOptions : pb::GeneratedMessage<CSharpMethodOptions, CSharpMethodOptions.Builder> {
-    private static readonly CSharpMethodOptions defaultInstance = new Builder().BuildPartial();
+    private static readonly CSharpMethodOptions defaultInstance = new CSharpMethodOptions().MakeReadOnly();
     private static readonly string[] _cSharpMethodOptionsFieldNames = new string[] { "dispatch_id" };
     private static readonly uint[] _cSharpMethodOptionsFieldTags = new uint[] { 8 };
     public static CSharpMethodOptions DefaultInstance {
@@ -1471,7 +1483,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpMethodOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpMethodOptions ThisMessage {
@@ -1557,6 +1569,10 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     public static CSharpMethodOptions ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private CSharpMethodOptions MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1572,22 +1588,22 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new CSharpMethodOptions();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(CSharpMethodOptions cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      CSharpMethodOptions result;
+      private bool resultIsReadOnly;
+      private CSharpMethodOptions result;
       
       private CSharpMethodOptions PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           CSharpMethodOptions original = result;
           result = new CSharpMethodOptions();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1602,13 +1618,13 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new CSharpMethodOptions();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1624,11 +1640,11 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
       }
       
       public override CSharpMethodOptions BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasFullProtoFile.cs

@@ -128,13 +128,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropPerson : pb::ExtendableMessage<TestInteropPerson, TestInteropPerson.Builder> {
-    private static readonly TestInteropPerson defaultInstance = new Builder().BuildPartial();
+    private static readonly TestInteropPerson defaultInstance = new TestInteropPerson().MakeReadOnly();
     public static TestInteropPerson DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestInteropPerson DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestInteropPerson ThisMessage {
@@ -166,13 +166,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class PhoneNumber : pb::GeneratedMessage<PhoneNumber, PhoneNumber.Builder> {
-        private static readonly PhoneNumber defaultInstance = new Builder().BuildPartial();
+        private static readonly PhoneNumber defaultInstance = new PhoneNumber().MakeReadOnly();
         public static PhoneNumber DefaultInstance {
           get { return defaultInstance; }
         }
         
         public override PhoneNumber DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override PhoneNumber ThisMessage {
@@ -237,6 +237,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static PhoneNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private PhoneNumber MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -252,22 +256,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new PhoneNumber();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(PhoneNumber cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          PhoneNumber result;
+          private bool resultIsReadOnly;
+          private PhoneNumber result;
           
           private PhoneNumber PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               PhoneNumber original = result;
               result = new PhoneNumber();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -282,13 +286,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new PhoneNumber();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -304,11 +308,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override PhoneNumber BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -362,13 +366,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Addresses : pb::GeneratedMessage<Addresses, Addresses.Builder> {
-        private static readonly Addresses defaultInstance = new Builder().BuildPartial();
+        private static readonly Addresses defaultInstance = new Addresses().MakeReadOnly();
         public static Addresses DefaultInstance {
           get { return defaultInstance; }
         }
         
         public override Addresses DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Addresses ThisMessage {
@@ -464,6 +468,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Addresses ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private Addresses MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -479,22 +487,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new Addresses();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(Addresses cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          Addresses result;
+          private bool resultIsReadOnly;
+          private Addresses result;
           
           private Addresses PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               Addresses original = result;
               result = new Addresses();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -509,13 +517,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new Addresses();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -531,11 +539,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Addresses BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -749,6 +757,13 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropPerson ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestInteropPerson MakeReadOnly() {
+      codes_.MakeReadOnly();
+      phone_.MakeReadOnly();
+      addresses_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -764,22 +779,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestInteropPerson();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestInteropPerson cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestInteropPerson result;
+      private bool resultIsReadOnly;
+      private TestInteropPerson result;
       
       private TestInteropPerson PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestInteropPerson original = result;
           result = new TestInteropPerson();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -794,13 +809,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestInteropPerson();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -816,14 +831,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestInteropPerson BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.codes_.MakeReadOnly();
-        result.phone_.MakeReadOnly();
-        result.addresses_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -1016,13 +1028,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropEmployeeId : pb::GeneratedMessage<TestInteropEmployeeId, TestInteropEmployeeId.Builder> {
-    private static readonly TestInteropEmployeeId defaultInstance = new Builder().BuildPartial();
+    private static readonly TestInteropEmployeeId defaultInstance = new TestInteropEmployeeId().MakeReadOnly();
     public static TestInteropEmployeeId DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestInteropEmployeeId DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestInteropEmployeeId ThisMessage {
@@ -1077,6 +1089,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropEmployeeId ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestInteropEmployeeId MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1092,22 +1108,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestInteropEmployeeId();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestInteropEmployeeId cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestInteropEmployeeId result;
+      private bool resultIsReadOnly;
+      private TestInteropEmployeeId result;
       
       private TestInteropEmployeeId PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestInteropEmployeeId original = result;
           result = new TestInteropEmployeeId();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1122,13 +1138,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestInteropEmployeeId();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1144,11 +1160,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestInteropEmployeeId BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -1182,13 +1198,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMissingFieldsA : pb::GeneratedMessage<TestMissingFieldsA, TestMissingFieldsA.Builder> {
-    private static readonly TestMissingFieldsA defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMissingFieldsA defaultInstance = new TestMissingFieldsA().MakeReadOnly();
     public static TestMissingFieldsA DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestMissingFieldsA DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMissingFieldsA ThisMessage {
@@ -1212,13 +1228,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class SubA : pb::GeneratedMessage<SubA, SubA.Builder> {
-        private static readonly SubA defaultInstance = new Builder().BuildPartial();
+        private static readonly SubA defaultInstance = new SubA().MakeReadOnly();
         public static SubA DefaultInstance {
           get { return defaultInstance; }
         }
         
         public override SubA DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override SubA ThisMessage {
@@ -1285,6 +1301,11 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static SubA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private SubA MakeReadOnly() {
+          values_.MakeReadOnly();
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1300,22 +1321,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new SubA();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(SubA cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          SubA result;
+          private bool resultIsReadOnly;
+          private SubA result;
           
           private SubA PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               SubA original = result;
               result = new SubA();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -1330,13 +1351,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new SubA();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -1352,12 +1373,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override SubA BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            result.values_.MakeReadOnly();
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -1453,12 +1473,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int TestAFieldNumber = 11;
     private bool hasTestA;
-    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA testA_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA testA_;
     public bool HasTestA {
       get { return hasTestA; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA TestA {
-      get { return testA_; }
+      get { return testA_ ?? global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance; }
     }
     
     public static TestMissingFieldsA ParseFrom(pb::ByteString data) {
@@ -1491,6 +1511,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMissingFieldsA ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMissingFieldsA MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1506,22 +1530,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMissingFieldsA();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMissingFieldsA cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMissingFieldsA result;
+      private bool resultIsReadOnly;
+      private TestMissingFieldsA result;
       
       private TestMissingFieldsA PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMissingFieldsA original = result;
           result = new TestMissingFieldsA();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1536,13 +1560,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMissingFieldsA();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1558,11 +1582,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMissingFieldsA BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -1664,7 +1688,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearTestA() {
         PrepareBuilder();
         result.hasTestA = false;
-        result.testA_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsA.Types.SubA.DefaultInstance;
+        result.testA_ = null;
         return this;
       }
     }
@@ -1677,13 +1701,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMissingFieldsB : pb::GeneratedMessage<TestMissingFieldsB, TestMissingFieldsB.Builder> {
-    private static readonly TestMissingFieldsB defaultInstance = new Builder().BuildPartial();
+    private static readonly TestMissingFieldsB defaultInstance = new TestMissingFieldsB().MakeReadOnly();
     public static TestMissingFieldsB DefaultInstance {
       get { return defaultInstance; }
     }
     
     public override TestMissingFieldsB DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMissingFieldsB ThisMessage {
@@ -1707,13 +1731,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class SubB : pb::GeneratedMessage<SubB, SubB.Builder> {
-        private static readonly SubB defaultInstance = new Builder().BuildPartial();
+        private static readonly SubB defaultInstance = new SubB().MakeReadOnly();
         public static SubB DefaultInstance {
           get { return defaultInstance; }
         }
         
         public override SubB DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override SubB ThisMessage {
@@ -1770,6 +1794,11 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static SubB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private SubB MakeReadOnly() {
+          values_.MakeReadOnly();
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1785,22 +1814,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new SubB();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(SubB cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          SubB result;
+          private bool resultIsReadOnly;
+          private SubB result;
           
           private SubB PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               SubB original = result;
               result = new SubB();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -1815,13 +1844,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new SubB();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -1837,12 +1866,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override SubB BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            result.values_.MakeReadOnly();
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -1918,12 +1946,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int TestBFieldNumber = 12;
     private bool hasTestB;
-    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB testB_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB testB_;
     public bool HasTestB {
       get { return hasTestB; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB TestB {
-      get { return testB_; }
+      get { return testB_ ?? global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance; }
     }
     
     public static TestMissingFieldsB ParseFrom(pb::ByteString data) {
@@ -1956,6 +1984,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestMissingFieldsB ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestMissingFieldsB MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1971,22 +2003,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestMissingFieldsB();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestMissingFieldsB cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestMissingFieldsB result;
+      private bool resultIsReadOnly;
+      private TestMissingFieldsB result;
       
       private TestMissingFieldsB PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestMissingFieldsB original = result;
           result = new TestMissingFieldsB();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -2001,13 +2033,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestMissingFieldsB();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -2023,11 +2055,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMissingFieldsB BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -2129,7 +2161,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearTestB() {
         PrepareBuilder();
         result.hasTestB = false;
-        result.testB_ = global::Google.ProtocolBuffers.TestProtos.TestMissingFieldsB.Types.SubB.DefaultInstance;
+        result.testB_ = null;
         return this;
       }
     }

src/ProtocolBuffersLite.Test/TestProtos/UnitTestExtrasLiteProtoFile.cs

@@ -59,7 +59,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestRequiredLite : pb::GeneratedMessageLite<TestRequiredLite, TestRequiredLite.Builder> {
-    private static readonly TestRequiredLite defaultInstance = new Builder().BuildPartial();
+    private static readonly TestRequiredLite defaultInstance = new TestRequiredLite().MakeReadOnly();
     private static readonly string[] _testRequiredLiteFieldNames = new string[] { "d", "en" };
     private static readonly uint[] _testRequiredLiteFieldTags = new uint[] { 8, 16 };
     public static TestRequiredLite DefaultInstance {
@@ -67,7 +67,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestRequiredLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestRequiredLite ThisMessage {
@@ -183,6 +183,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestRequiredLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestRequiredLite MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -198,22 +202,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestRequiredLite();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestRequiredLite cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestRequiredLite result;
+      private bool resultIsReadOnly;
+      private TestRequiredLite result;
       
       private TestRequiredLite PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestRequiredLite original = result;
           result = new TestRequiredLite();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -228,13 +232,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestRequiredLite();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -246,11 +250,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestRequiredLite BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessageLite other) {
@@ -371,7 +375,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropPersonLite : pb::ExtendableMessageLite<TestInteropPersonLite, TestInteropPersonLite.Builder> {
-    private static readonly TestInteropPersonLite defaultInstance = new Builder().BuildPartial();
+    private static readonly TestInteropPersonLite defaultInstance = new TestInteropPersonLite().MakeReadOnly();
     private static readonly string[] _testInteropPersonLiteFieldNames = new string[] { "addresses", "codes", "email", "id", "name", "phone" };
     private static readonly uint[] _testInteropPersonLiteFieldTags = new uint[] { 43, 82, 26, 16, 10, 34 };
     public static TestInteropPersonLite DefaultInstance {
@@ -379,7 +383,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestInteropPersonLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestInteropPersonLite ThisMessage {
@@ -403,7 +407,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class PhoneNumber : pb::GeneratedMessageLite<PhoneNumber, PhoneNumber.Builder> {
-        private static readonly PhoneNumber defaultInstance = new Builder().BuildPartial();
+        private static readonly PhoneNumber defaultInstance = new PhoneNumber().MakeReadOnly();
         private static readonly string[] _phoneNumberFieldNames = new string[] { "number", "type" };
         private static readonly uint[] _phoneNumberFieldTags = new uint[] { 10, 16 };
         public static PhoneNumber DefaultInstance {
@@ -411,7 +415,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override PhoneNumber DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override PhoneNumber ThisMessage {
@@ -526,6 +530,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static PhoneNumber ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private PhoneNumber MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -541,22 +549,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new PhoneNumber();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(PhoneNumber cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          PhoneNumber result;
+          private bool resultIsReadOnly;
+          private PhoneNumber result;
           
           private PhoneNumber PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               PhoneNumber original = result;
               result = new PhoneNumber();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -571,13 +579,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new PhoneNumber();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -589,11 +597,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override PhoneNumber BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessageLite other) {
@@ -715,7 +723,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Addresses : pb::GeneratedMessageLite<Addresses, Addresses.Builder> {
-        private static readonly Addresses defaultInstance = new Builder().BuildPartial();
+        private static readonly Addresses defaultInstance = new Addresses().MakeReadOnly();
         private static readonly string[] _addressesFieldNames = new string[] { "address", "address2", "city", "state", "zip" };
         private static readonly uint[] _addressesFieldTags = new uint[] { 10, 18, 26, 34, 45 };
         public static Addresses DefaultInstance {
@@ -723,7 +731,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Addresses DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Addresses ThisMessage {
@@ -899,6 +907,10 @@ namespace Google.ProtocolBuffers.TestProtos {
         public static Addresses ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
           return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
         }
+        private Addresses MakeReadOnly() {
+          return this;
+        }
+        
         public static Builder CreateBuilder() { return new Builder(); }
         public override Builder ToBuilder() { return CreateBuilder(this); }
         public override Builder CreateBuilderForType() { return new Builder(); }
@@ -914,22 +926,22 @@ namespace Google.ProtocolBuffers.TestProtos {
             get { return this; }
           }
           public Builder() {
-            result = DefaultInstance ?? new Addresses();
-            builderIsReadOnly = result == DefaultInstance;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
           }
           internal Builder(Addresses cloneFrom) {
             result = cloneFrom;
-            builderIsReadOnly = true;
+            resultIsReadOnly = true;
           }
           
-          bool builderIsReadOnly;
-          Addresses result;
+          private bool resultIsReadOnly;
+          private Addresses result;
           
           private Addresses PrepareBuilder() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               Addresses original = result;
               result = new Addresses();
-              builderIsReadOnly = false;
+              resultIsReadOnly = false;
               MergeFrom(original);
             }
             return result;
@@ -944,13 +956,13 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder Clear() {
-            result = DefaultInstance ?? new Addresses();
-            builderIsReadOnly = true;
+            result = DefaultInstance;
+            resultIsReadOnly = true;
             return this;
           }
           
           public override Builder Clone() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return new Builder(result);
             } else {
               return new Builder().MergeFrom(result);
@@ -962,11 +974,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Addresses BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           public override Builder MergeFrom(pb::IMessageLite other) {
@@ -1388,6 +1400,13 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropPersonLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestInteropPersonLite MakeReadOnly() {
+      codes_.MakeReadOnly();
+      phone_.MakeReadOnly();
+      addresses_.MakeReadOnly();
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1403,22 +1422,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestInteropPersonLite();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestInteropPersonLite cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestInteropPersonLite result;
+      private bool resultIsReadOnly;
+      private TestInteropPersonLite result;
       
       private TestInteropPersonLite PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestInteropPersonLite original = result;
           result = new TestInteropPersonLite();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1433,13 +1452,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestInteropPersonLite();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1451,14 +1470,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestInteropPersonLite BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        result.codes_.MakeReadOnly();
-        result.phone_.MakeReadOnly();
-        result.addresses_.MakeReadOnly();
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessageLite other) {
@@ -1745,7 +1761,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropEmployeeIdLite : pb::GeneratedMessageLite<TestInteropEmployeeIdLite, TestInteropEmployeeIdLite.Builder> {
-    private static readonly TestInteropEmployeeIdLite defaultInstance = new Builder().BuildPartial();
+    private static readonly TestInteropEmployeeIdLite defaultInstance = new TestInteropEmployeeIdLite().MakeReadOnly();
     private static readonly string[] _testInteropEmployeeIdLiteFieldNames = new string[] { "number" };
     private static readonly uint[] _testInteropEmployeeIdLiteFieldTags = new uint[] { 10 };
     public static TestInteropEmployeeIdLite DefaultInstance {
@@ -1753,7 +1769,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestInteropEmployeeIdLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestInteropEmployeeIdLite ThisMessage {
@@ -1849,6 +1865,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestInteropEmployeeIdLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestInteropEmployeeIdLite MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -1864,22 +1884,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestInteropEmployeeIdLite();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestInteropEmployeeIdLite cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestInteropEmployeeIdLite result;
+      private bool resultIsReadOnly;
+      private TestInteropEmployeeIdLite result;
       
       private TestInteropEmployeeIdLite PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestInteropEmployeeIdLite original = result;
           result = new TestInteropEmployeeIdLite();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -1894,13 +1914,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestInteropEmployeeIdLite();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -1912,11 +1932,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestInteropEmployeeIdLite BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessageLite other) {

src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportLiteProtoFile.cs

@@ -43,7 +43,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessageLite : pb::GeneratedMessageLite<ImportMessageLite, ImportMessageLite.Builder> {
-    private static readonly ImportMessageLite defaultInstance = new Builder().BuildPartial();
+    private static readonly ImportMessageLite defaultInstance = new ImportMessageLite().MakeReadOnly();
     private static readonly string[] _importMessageLiteFieldNames = new string[] { "d" };
     private static readonly uint[] _importMessageLiteFieldTags = new uint[] { 8 };
     public static ImportMessageLite DefaultInstance {
@@ -51,7 +51,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessageLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessageLite ThisMessage {
@@ -146,6 +146,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessageLite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private ImportMessageLite MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -161,22 +165,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new ImportMessageLite();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(ImportMessageLite cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      ImportMessageLite result;
+      private bool resultIsReadOnly;
+      private ImportMessageLite result;
       
       private ImportMessageLite PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           ImportMessageLite original = result;
           result = new ImportMessageLite();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -191,13 +195,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new ImportMessageLite();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -209,11 +213,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override ImportMessageLite BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessageLite other) {

src/ProtocolBuffersLite.Test/TestProtos/UnitTestImportProtoFile.cs

@@ -71,7 +71,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessage : pb::GeneratedMessage<ImportMessage, ImportMessage.Builder> {
-    private static readonly ImportMessage defaultInstance = new Builder().BuildPartial();
+    private static readonly ImportMessage defaultInstance = new ImportMessage().MakeReadOnly();
     private static readonly string[] _importMessageFieldNames = new string[] { "d" };
     private static readonly uint[] _importMessageFieldTags = new uint[] { 8 };
     public static ImportMessage DefaultInstance {
@@ -79,7 +79,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessage ThisMessage {
@@ -165,6 +165,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static ImportMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private ImportMessage MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -180,22 +184,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new ImportMessage();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(ImportMessage cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      ImportMessage result;
+      private bool resultIsReadOnly;
+      private ImportMessage result;
       
       private ImportMessage PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           ImportMessage original = result;
           result = new ImportMessage();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -210,13 +214,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new ImportMessage();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -232,11 +236,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override ImportMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessage other) {

src/ProtocolBuffersLite.Test/TestProtos/UnitTestLiteImportNonLiteProtoFile.cs

@@ -32,7 +32,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestLiteImportsNonlite : pb::GeneratedMessageLite<TestLiteImportsNonlite, TestLiteImportsNonlite.Builder> {
-    private static readonly TestLiteImportsNonlite defaultInstance = new Builder().BuildPartial();
+    private static readonly TestLiteImportsNonlite defaultInstance = new TestLiteImportsNonlite().MakeReadOnly();
     private static readonly string[] _testLiteImportsNonliteFieldNames = new string[] { "message" };
     private static readonly uint[] _testLiteImportsNonliteFieldTags = new uint[] { 10 };
     public static TestLiteImportsNonlite DefaultInstance {
@@ -40,7 +40,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestLiteImportsNonlite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestLiteImportsNonlite ThisMessage {
@@ -49,12 +49,12 @@ namespace Google.ProtocolBuffers.TestProtos {
     
     public const int MessageFieldNumber = 1;
     private bool hasMessage;
-    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance;
+    private global::Google.ProtocolBuffers.TestProtos.TestAllTypes message_;
     public bool HasMessage {
       get { return hasMessage; }
     }
     public global::Google.ProtocolBuffers.TestProtos.TestAllTypes Message {
-      get { return message_; }
+      get { return message_ ?? global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance; }
     }
     
     public override bool IsInitialized {
@@ -135,6 +135,10 @@ namespace Google.ProtocolBuffers.TestProtos {
     public static TestLiteImportsNonlite ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();
     }
+    private TestLiteImportsNonlite MakeReadOnly() {
+      return this;
+    }
+    
     public static Builder CreateBuilder() { return new Builder(); }
     public override Builder ToBuilder() { return CreateBuilder(this); }
     public override Builder CreateBuilderForType() { return new Builder(); }
@@ -150,22 +154,22 @@ namespace Google.ProtocolBuffers.TestProtos {
         get { return this; }
       }
       public Builder() {
-        result = DefaultInstance ?? new TestLiteImportsNonlite();
-        builderIsReadOnly = result == DefaultInstance;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
       }
       internal Builder(TestLiteImportsNonlite cloneFrom) {
         result = cloneFrom;
-        builderIsReadOnly = true;
+        resultIsReadOnly = true;
       }
       
-      bool builderIsReadOnly;
-      TestLiteImportsNonlite result;
+      private bool resultIsReadOnly;
+      private TestLiteImportsNonlite result;
       
       private TestLiteImportsNonlite PrepareBuilder() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           TestLiteImportsNonlite original = result;
           result = new TestLiteImportsNonlite();
-          builderIsReadOnly = false;
+          resultIsReadOnly = false;
           MergeFrom(original);
         }
         return result;
@@ -180,13 +184,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder Clear() {
-        result = DefaultInstance ?? new TestLiteImportsNonlite();
-        builderIsReadOnly = true;
+        result = DefaultInstance;
+        resultIsReadOnly = true;
         return this;
       }
       
       public override Builder Clone() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return new Builder(result);
         } else {
           return new Builder().MergeFrom(result);
@@ -198,11 +202,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestLiteImportsNonlite BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       public override Builder MergeFrom(pb::IMessageLite other) {
@@ -304,7 +308,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessage() {
         PrepareBuilder();
         result.hasMessage = false;
-        result.message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance;
+        result.message_ = null;
         return this;
       }
     }