merged issue-28

protos/extest/unittest_issues.proto

@@ -86,6 +86,15 @@ message NumberField {
     optional int32 _01 = 1;
 }
 
+// Issue 28: Circular message dependencies result in null defaults for DefaultInstance
+
+message MyMessageAReferenceB {
+    required MyMessageBReferenceA value = 1;
+}
+
+message MyMessageBReferenceA {
+    required MyMessageAReferenceB value = 1;
+}
 
 // issue 19 - negative enum values
 

src/AddressBook/AddressBookProtos.cs

@@ -69,7 +69,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class Person : pb::GeneratedMessage<Person, Person.Builder> {
     private Person() { }
-    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 {
@@ -77,7 +77,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     }
     
     public override Person DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override Person ThisMessage {
@@ -110,7 +110,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class PhoneNumber : pb::GeneratedMessage<PhoneNumber, PhoneNumber.Builder> {
         private PhoneNumber() { }
-        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 {
@@ -118,7 +118,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
         }
         
         public override PhoneNumber DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override PhoneNumber ThisMessage {
@@ -221,6 +221,10 @@ 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(); }
@@ -236,22 +240,22 @@ 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;
@@ -266,13 +270,13 @@ 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);
@@ -288,11 +292,11 @@ 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) {
@@ -557,6 +561,11 @@ 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(); }
@@ -572,22 +581,22 @@ 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;
@@ -602,13 +611,13 @@ 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);
@@ -624,12 +633,11 @@ 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) {
@@ -841,7 +849,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class AddressBook : pb::GeneratedMessage<AddressBook, AddressBook.Builder> {
     private AddressBook() { }
-    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 {
@@ -849,7 +857,7 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
     }
     
     public override AddressBook DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override AddressBook ThisMessage {
@@ -940,6 +948,11 @@ 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(); }
@@ -955,22 +968,22 @@ 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;
@@ -985,13 +998,13 @@ 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);
@@ -1007,12 +1020,11 @@ 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/ProtoGen/MessageFieldGenerator.cs

@@ -48,14 +48,14 @@ namespace Google.ProtocolBuffers.ProtoGen
         public void GenerateMembers(TextGenerator writer)
         {
             writer.WriteLine("private bool has{0};", PropertyName);
-            writer.WriteLine("private {0} {1}_ = {2};", TypeName, Name, DefaultValue);
+            writer.WriteLine("private {0} {1}_;", TypeName, Name);
             AddDeprecatedFlag(writer);
             writer.WriteLine("public bool Has{0} {{", PropertyName);
             writer.WriteLine("  get {{ return has{0}; }}", PropertyName);
             writer.WriteLine("}");
             AddDeprecatedFlag(writer);
             writer.WriteLine("public {0} {1} {{", TypeName, PropertyName);
-            writer.WriteLine("  get {{ return {0}_; }}", Name);
+            writer.WriteLine("  get {{ return {0}_ ?? {1}; }}", Name, DefaultValue);
             writer.WriteLine("}");
         }
 
@@ -104,7 +104,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("public Builder Clear{0}() {{", PropertyName);
             writer.WriteLine("  PrepareBuilder();");
             writer.WriteLine("  result.has{0} = false;", PropertyName);
-            writer.WriteLine("  result.{0}_ = {1};", Name, DefaultValue);
+            writer.WriteLine("  result.{0}_ = null;", Name);
             writer.WriteLine("  return this;");
             writer.WriteLine("}");
         }

src/ProtoGen/MessageGenerator.cs

@@ -183,7 +183,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.Indent();
             writer.WriteLine("private {0}() {{ }}", ClassName);
             // Must call BuildPartial() to make sure all lists are made read-only
-            writer.WriteLine("private static readonly {0} defaultInstance = new Builder().BuildPartial();", ClassName);
+            writer.WriteLine("private static readonly {0} defaultInstance = new {0}().MakeReadOnly();", ClassName);
 
             if (OptimizeSpeed)
             {
@@ -204,7 +204,7 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine("}");
             writer.WriteLine();
             writer.WriteLine("public override {0} DefaultInstanceForType {{", ClassName);
-            writer.WriteLine("  get { return defaultInstance; }");
+            writer.WriteLine("  get { return DefaultInstance; }");
             writer.WriteLine("}");
             writer.WriteLine();
             writer.WriteLine("protected override {0} ThisMessage {{", ClassName);
@@ -552,6 +552,17 @@ namespace Google.ProtocolBuffers.ProtoGen
 
         private void GenerateBuilder(TextGenerator writer)
         {
+            writer.WriteLine("private {0} MakeReadOnly() {{", ClassName);
+            writer.Indent();
+            foreach (FieldDescriptor field in Descriptor.Fields)
+            {
+                CreateFieldGenerator(field).GenerateBuildingCode(writer);
+            }
+            writer.WriteLine("return this;");
+            writer.Outdent();
+            writer.WriteLine("}");
+            writer.WriteLine();
+
             writer.WriteLine("public static Builder CreateBuilder() { return new Builder(); }");
             writer.WriteLine("public override Builder ToBuilder() { return CreateBuilder(this); }");
             writer.WriteLine("public override Builder CreateBuilderForType() { return new Builder(); }");
@@ -594,23 +605,23 @@ namespace Google.ProtocolBuffers.ProtoGen
             //default constructor
             writer.WriteLine("public Builder() {");
             //Durring static initialization of message, DefaultInstance is expected to return null.
-            writer.WriteLine("  result = DefaultInstance ?? new {0}();", ClassName);
-            writer.WriteLine("  builderIsReadOnly = result == DefaultInstance;");
+            writer.WriteLine("  result = DefaultInstance;");
+            writer.WriteLine("  resultIsReadOnly = true;");
             writer.WriteLine("}");
             //clone constructor
             writer.WriteLine("internal Builder({0} cloneFrom) {{", ClassName);
             writer.WriteLine("  result = cloneFrom;");
-            writer.WriteLine("  builderIsReadOnly = true;");
+            writer.WriteLine("  resultIsReadOnly = true;");
             writer.WriteLine("}");
             writer.WriteLine();
-            writer.WriteLine("bool builderIsReadOnly;");
-            writer.WriteLine("{0} result;", ClassName);
+            writer.WriteLine("private bool resultIsReadOnly;");
+            writer.WriteLine("private {0} result;", ClassName);
             writer.WriteLine();
             writer.WriteLine("private {0} PrepareBuilder() {{", ClassName);
-            writer.WriteLine("  if (builderIsReadOnly) {");
+            writer.WriteLine("  if (resultIsReadOnly) {");
             writer.WriteLine("    {0} original = result;", ClassName);
             writer.WriteLine("    result = new {0}();", ClassName);
-            writer.WriteLine("    builderIsReadOnly = false;");
+            writer.WriteLine("    resultIsReadOnly = false;");
             writer.WriteLine("    MergeFrom(original);");
             writer.WriteLine("  }");
             writer.WriteLine("  return result;");
@@ -626,13 +637,13 @@ namespace Google.ProtocolBuffers.ProtoGen
             writer.WriteLine();
             //Not actually expecting that DefaultInstance would ever be null here; however, we will ensure it does not break
             writer.WriteLine("public override Builder Clear() {");
-            writer.WriteLine("  result = DefaultInstance ?? new {0}();", ClassName);
-            writer.WriteLine("  builderIsReadOnly = true;");
+            writer.WriteLine("  result = DefaultInstance;", ClassName);
+            writer.WriteLine("  resultIsReadOnly = true;");
             writer.WriteLine("  return this;");
             writer.WriteLine("}");
             writer.WriteLine();
             writer.WriteLine("public override Builder Clone() {");
-            writer.WriteLine("  if (builderIsReadOnly) {");
+            writer.WriteLine("  if (resultIsReadOnly) {");
             writer.WriteLine("    return new Builder(result);");
             writer.WriteLine("  } else {");
             writer.WriteLine("    return new Builder().MergeFrom(result);");
@@ -653,15 +664,11 @@ namespace Google.ProtocolBuffers.ProtoGen
 
             writer.WriteLine("public override {0} BuildPartial() {{", ClassName);
             writer.Indent();
-            writer.WriteLine("if (builderIsReadOnly) {");
+            writer.WriteLine("if (resultIsReadOnly) {");
             writer.WriteLine("  return result;");
             writer.WriteLine("}");
-            foreach (FieldDescriptor field in Descriptor.Fields)
-            {
-                CreateFieldGenerator(field).GenerateBuildingCode(writer);
-            }
-            writer.WriteLine("builderIsReadOnly = true;");
-            writer.WriteLine("return result;");
+            writer.WriteLine("resultIsReadOnly = true;");
+            writer.WriteLine("return result.MakeReadOnly();");
             writer.Outdent();
             writer.WriteLine("}");
             writer.WriteLine();

src/ProtoGen/RepeatedEnumFieldGenerator.cs

@@ -120,7 +120,7 @@ namespace Google.ProtocolBuffers.ProtoGen
 
         public void GenerateBuildingCode(TextGenerator writer)
         {
-            writer.WriteLine("result.{0}_.MakeReadOnly();", Name);
+            writer.WriteLine("{0}_.MakeReadOnly();", Name);
         }
 
         public void GenerateParsingCode(TextGenerator writer)

src/ProtoGen/RepeatedMessageFieldGenerator.cs

@@ -134,7 +134,7 @@ namespace Google.ProtocolBuffers.ProtoGen
 
         public void GenerateBuildingCode(TextGenerator writer)
         {
-            writer.WriteLine("result.{0}_.MakeReadOnly();", Name);
+            writer.WriteLine("{0}_.MakeReadOnly();", Name);
         }
 
         public void GenerateParsingCode(TextGenerator writer)

src/ProtoGen/RepeatedPrimitiveFieldGenerator.cs

@@ -122,7 +122,7 @@ namespace Google.ProtocolBuffers.ProtoGen
 
         public void GenerateBuildingCode(TextGenerator writer)
         {
-            writer.WriteLine("result.{0}_.MakeReadOnly();", Name);
+            writer.WriteLine("{0}_.MakeReadOnly();", Name);
         }
 
         public void GenerateParsingCode(TextGenerator writer)

src/ProtocolBuffers.Test/ReusableBuilderTest.cs

@@ -1,16 +1,29 @@
 using System;
 using System.Collections.Generic;
+using System.ComponentModel;
 using System.Text;
 using Google.ProtocolBuffers.Collections;
 using NUnit.Framework;
 using Google.ProtocolBuffers.TestProtos;
 using Google.ProtocolBuffers.Serialization;
+using UnitTest.Issues.TestProtos;
 
 namespace Google.ProtocolBuffers
 {
     [TestFixture]
     public class ReusableBuilderTest
     {
+        [Test, Description("Issue 28: Circular message dependencies result in null defaults for DefaultInstance")]
+        public void EnsureStaticCicularReference()
+        {
+            MyMessageAReferenceB ab = MyMessageAReferenceB.DefaultInstance;
+            Assert.IsNotNull(ab);
+            Assert.IsNotNull(ab.Value);
+            MyMessageBReferenceA ba = MyMessageBReferenceA.DefaultInstance;
+            Assert.IsNotNull(ba);
+            Assert.IsNotNull(ba.Value);
+        }
+
         [Test]
         public void TestModifyDefaultInstance()
         {

src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs

@@ -61,7 +61,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class OptionsMessage : pb::GeneratedMessage<OptionsMessage, OptionsMessage.Builder> {
     private OptionsMessage() { }
-    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 {
@@ -69,7 +69,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override OptionsMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override OptionsMessage ThisMessage {
@@ -187,6 +187,10 @@ 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(); }
@@ -202,22 +206,22 @@ 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;
@@ -232,13 +236,13 @@ 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);
@@ -254,11 +258,11 @@ 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

@@ -65,7 +65,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestEmbedOptimizedForSize : pb::GeneratedMessage<TestEmbedOptimizedForSize, TestEmbedOptimizedForSize.Builder> {
     private TestEmbedOptimizedForSize() { }
-    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 {
@@ -73,7 +73,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestEmbedOptimizedForSize DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestEmbedOptimizedForSize ThisMessage {
@@ -90,12 +90,12 @@ 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;
@@ -183,6 +183,11 @@ 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(); }
@@ -198,22 +203,22 @@ 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;
@@ -228,13 +233,13 @@ 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);
@@ -250,12 +255,11 @@ 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) {
@@ -378,7 +382,7 @@ 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/UnitTestGoogleSizeProtoFile.cs

@@ -138,13 +138,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage1 : pb::GeneratedMessage<SizeMessage1, SizeMessage1.Builder> {
     private SizeMessage1() { }
-    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 {
@@ -505,12 +505,12 @@ 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;
@@ -603,6 +603,11 @@ 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(); }
@@ -618,22 +623,22 @@ 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;
@@ -648,13 +653,13 @@ 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);
@@ -670,12 +675,11 @@ 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();
       }
       
       
@@ -1417,7 +1421,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearField15() {
         PrepareBuilder();
         result.hasField15 = false;
-        result.field15_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage1SubMessage.DefaultInstance;
+        result.field15_ = null;
         return this;
       }
       
@@ -1552,13 +1556,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage1SubMessage : pb::GeneratedMessage<SizeMessage1SubMessage, SizeMessage1SubMessage.Builder> {
     private SizeMessage1SubMessage() { }
-    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 {
@@ -1807,6 +1811,10 @@ 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(); }
@@ -1822,22 +1830,22 @@ 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;
@@ -1852,13 +1860,13 @@ 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);
@@ -1874,11 +1882,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override SizeMessage1SubMessage BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -2302,13 +2310,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage2 : pb::GeneratedMessage<SizeMessage2, SizeMessage2.Builder> {
     private SizeMessage2() { }
-    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 {
@@ -2333,13 +2341,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Group1 : pb::GeneratedMessage<Group1, Group1.Builder> {
         private Group1() { }
-        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 {
@@ -2513,12 +2521,12 @@ 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) {
@@ -2551,6 +2559,13 @@ 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(); }
@@ -2566,22 +2581,22 @@ 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;
@@ -2596,13 +2611,13 @@ 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);
@@ -2618,14 +2633,11 @@ 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();
           }
           
           
@@ -3007,7 +3019,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           public Builder ClearField31() {
             PrepareBuilder();
             result.hasField31 = false;
-            result.field31_ = global::Google.ProtocolBuffers.TestProtos.SizeMessage2GroupedMessage.DefaultInstance;
+            result.field31_ = null;
             return this;
           }
         }
@@ -3357,6 +3369,14 @@ 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(); }
@@ -3372,22 +3392,22 @@ 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;
@@ -3402,13 +3422,13 @@ 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);
@@ -3424,15 +3444,11 @@ 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();
       }
       
       
@@ -4107,13 +4123,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SizeMessage2GroupedMessage : pb::GeneratedMessage<SizeMessage2GroupedMessage, SizeMessage2GroupedMessage.Builder> {
     private SizeMessage2GroupedMessage() { }
-    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 {
@@ -4268,6 +4284,10 @@ 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(); }
@@ -4283,22 +4303,22 @@ 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;
@@ -4313,13 +4333,13 @@ 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);
@@ -4335,11 +4355,11 @@ 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

@@ -138,7 +138,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage1 : pb::GeneratedMessage<SpeedMessage1, SpeedMessage1.Builder> {
     private SpeedMessage1() { }
-    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 {
@@ -146,7 +146,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage1 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage1 ThisMessage {
@@ -507,12 +507,12 @@ 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;
@@ -882,6 +882,11 @@ 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(); }
@@ -897,22 +902,22 @@ 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;
@@ -927,13 +932,13 @@ 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);
@@ -949,12 +954,11 @@ 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) {
@@ -2053,7 +2057,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearField15() {
         PrepareBuilder();
         result.hasField15 = false;
-        result.field15_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage1SubMessage.DefaultInstance;
+        result.field15_ = null;
         return this;
       }
       
@@ -2188,7 +2192,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage1SubMessage : pb::GeneratedMessage<SpeedMessage1SubMessage, SpeedMessage1SubMessage.Builder> {
     private SpeedMessage1SubMessage() { }
-    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 {
@@ -2196,7 +2200,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage1SubMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage1SubMessage ThisMessage {
@@ -2590,6 +2594,10 @@ 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(); }
@@ -2605,22 +2613,22 @@ 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;
@@ -2635,13 +2643,13 @@ 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);
@@ -2657,11 +2665,11 @@ 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) {
@@ -3289,7 +3297,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage2 : pb::GeneratedMessage<SpeedMessage2, SpeedMessage2.Builder> {
     private SpeedMessage2() { }
-    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 {
@@ -3297,7 +3305,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage2 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage2 ThisMessage {
@@ -3322,7 +3330,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Group1 : pb::GeneratedMessage<Group1, Group1.Builder> {
         private Group1() { }
-        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 {
@@ -3330,7 +3338,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Group1 DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Group1 ThisMessage {
@@ -3504,12 +3512,12 @@ 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 {
@@ -3680,6 +3688,13 @@ 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(); }
@@ -3695,22 +3710,22 @@ 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;
@@ -3725,13 +3740,13 @@ 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);
@@ -3747,14 +3762,11 @@ 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) {
@@ -4318,7 +4330,7 @@ namespace Google.ProtocolBuffers.TestProtos {
           public Builder ClearField31() {
             PrepareBuilder();
             result.hasField31 = false;
-            result.field31_ = global::Google.ProtocolBuffers.TestProtos.SpeedMessage2GroupedMessage.DefaultInstance;
+            result.field31_ = null;
             return this;
           }
         }
@@ -4888,6 +4900,14 @@ 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(); }
@@ -4903,22 +4923,22 @@ 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;
@@ -4933,13 +4953,13 @@ 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);
@@ -4955,15 +4975,11 @@ 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) {
@@ -5913,7 +5929,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SpeedMessage2GroupedMessage : pb::GeneratedMessage<SpeedMessage2GroupedMessage, SpeedMessage2GroupedMessage.Builder> {
     private SpeedMessage2GroupedMessage() { }
-    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 {
@@ -5921,7 +5937,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SpeedMessage2GroupedMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SpeedMessage2GroupedMessage ThisMessage {
@@ -6167,6 +6183,10 @@ 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(); }
@@ -6182,22 +6202,22 @@ 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;
@@ -6212,13 +6232,13 @@ 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);
@@ -6234,11 +6254,11 @@ 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

@@ -44,7 +44,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessageLite : pb::GeneratedMessageLite<ImportMessageLite, ImportMessageLite.Builder> {
     private ImportMessageLite() { }
-    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 {
@@ -52,7 +52,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessageLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessageLite ThisMessage {
@@ -147,6 +147,10 @@ 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(); }
@@ -162,22 +166,22 @@ 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;
@@ -192,13 +196,13 @@ 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);
@@ -210,11 +214,11 @@ 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

@@ -72,7 +72,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessage : pb::GeneratedMessage<ImportMessage, ImportMessage.Builder> {
     private ImportMessage() { }
-    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 {
@@ -80,7 +80,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessage ThisMessage {
@@ -166,6 +166,10 @@ 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(); }
@@ -181,22 +185,22 @@ 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;
@@ -211,13 +215,13 @@ 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);
@@ -233,11 +237,11 @@ 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

@@ -104,7 +104,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSet : pb::ExtendableMessage<TestMessageSet, TestMessageSet.Builder> {
     private TestMessageSet() { }
-    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 {
@@ -112,7 +112,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSet DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSet ThisMessage {
@@ -186,6 +186,10 @@ 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(); }
@@ -201,22 +205,22 @@ 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;
@@ -231,13 +235,13 @@ 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);
@@ -253,11 +257,11 @@ 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) {
@@ -336,7 +340,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSetContainer : pb::GeneratedMessage<TestMessageSetContainer, TestMessageSetContainer.Builder> {
     private TestMessageSetContainer() { }
-    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 {
@@ -344,7 +348,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSetContainer DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSetContainer ThisMessage {
@@ -361,12 +365,12 @@ 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 {
@@ -430,6 +434,10 @@ 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(); }
@@ -445,22 +453,22 @@ 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;
@@ -475,13 +483,13 @@ 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);
@@ -497,11 +505,11 @@ 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) {
@@ -617,7 +625,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessageSet() {
         PrepareBuilder();
         result.hasMessageSet = false;
-        result.messageSet_ = global::Google.ProtocolBuffers.TestProtos.TestMessageSet.DefaultInstance;
+        result.messageSet_ = null;
         return this;
       }
     }
@@ -631,7 +639,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSetExtension1 : pb::GeneratedMessage<TestMessageSetExtension1, TestMessageSetExtension1.Builder> {
     private TestMessageSetExtension1() { }
-    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 {
@@ -639,7 +647,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSetExtension1 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSetExtension1 ThisMessage {
@@ -727,6 +735,10 @@ 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(); }
@@ -742,22 +754,22 @@ 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;
@@ -772,13 +784,13 @@ 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);
@@ -794,11 +806,11 @@ 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) {
@@ -903,7 +915,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessageSetExtension2 : pb::GeneratedMessage<TestMessageSetExtension2, TestMessageSetExtension2.Builder> {
     private TestMessageSetExtension2() { }
-    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 {
@@ -911,7 +923,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestMessageSetExtension2 DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessageSetExtension2 ThisMessage {
@@ -999,6 +1011,10 @@ 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(); }
@@ -1014,22 +1030,22 @@ 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;
@@ -1044,13 +1060,13 @@ 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);
@@ -1066,11 +1082,11 @@ 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) {
@@ -1176,7 +1192,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class RawMessageSet : pb::GeneratedMessage<RawMessageSet, RawMessageSet.Builder> {
     private RawMessageSet() { }
-    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 {
@@ -1184,7 +1200,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override RawMessageSet DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override RawMessageSet ThisMessage {
@@ -1209,7 +1225,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Item : pb::GeneratedMessage<Item, Item.Builder> {
         private Item() { }
-        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 {
@@ -1217,7 +1233,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Item DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Item ThisMessage {
@@ -1321,6 +1337,10 @@ 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(); }
@@ -1336,22 +1356,22 @@ 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;
@@ -1366,13 +1386,13 @@ 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);
@@ -1388,11 +1408,11 @@ 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) {
@@ -1596,6 +1616,11 @@ 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(); }
@@ -1611,22 +1636,22 @@ 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;
@@ -1641,13 +1666,13 @@ 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);
@@ -1663,12 +1688,11 @@ 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

@@ -82,7 +82,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMessage : pb::ExtendableMessage<TestMessage, TestMessage.Builder> {
     private TestMessage() { }
-    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 {
@@ -90,7 +90,7 @@ namespace Google.ProtocolBuffers.TestProtos.NoGenericService {
     }
     
     public override TestMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestMessage ThisMessage {
@@ -180,6 +180,10 @@ 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(); }
@@ -195,22 +199,22 @@ 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;
@@ -225,13 +229,13 @@ 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);
@@ -247,11 +251,11 @@ 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

@@ -86,13 +86,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestOptimizedForSize : pb::ExtendableMessage<TestOptimizedForSize, TestOptimizedForSize.Builder> {
     private TestOptimizedForSize() { }
-    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 {
@@ -123,12 +123,12 @@ 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) {
@@ -161,6 +161,10 @@ 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(); }
@@ -176,22 +180,22 @@ 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;
@@ -206,13 +210,13 @@ 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);
@@ -228,11 +232,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -292,7 +296,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMsg() {
         PrepareBuilder();
         result.hasMsg = false;
-        result.msg_ = global::Google.ProtocolBuffers.TestProtos.ForeignMessage.DefaultInstance;
+        result.msg_ = null;
         return this;
       }
     }
@@ -306,13 +310,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestRequiredOptimizedForSize : pb::GeneratedMessage<TestRequiredOptimizedForSize, TestRequiredOptimizedForSize.Builder> {
     private TestRequiredOptimizedForSize() { }
-    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 {
@@ -367,6 +371,10 @@ 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(); }
@@ -382,22 +390,22 @@ 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;
@@ -412,13 +420,13 @@ 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);
@@ -434,11 +442,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestRequiredOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -472,13 +480,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestOptionalOptimizedForSize : pb::GeneratedMessage<TestOptionalOptimizedForSize, TestOptionalOptimizedForSize.Builder> {
     private TestOptionalOptimizedForSize() { }
-    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 {
@@ -495,12 +503,12 @@ 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) {
@@ -533,6 +541,10 @@ 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(); }
@@ -548,22 +560,22 @@ 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;
@@ -578,13 +590,13 @@ 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);
@@ -600,11 +612,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestOptionalOptimizedForSize BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -644,7 +656,7 @@ 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

@@ -84,7 +84,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SearchRequest : pb::GeneratedMessage<SearchRequest, SearchRequest.Builder> {
     private SearchRequest() { }
-    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 {
@@ -92,7 +92,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SearchRequest DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SearchRequest ThisMessage {
@@ -185,6 +185,11 @@ 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(); }
@@ -200,22 +205,22 @@ 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;
@@ -230,13 +235,13 @@ 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);
@@ -252,12 +257,11 @@ 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) {
@@ -374,7 +378,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class SearchResponse : pb::GeneratedMessage<SearchResponse, SearchResponse.Builder> {
     private SearchResponse() { }
-    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 {
@@ -382,7 +386,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override SearchResponse DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override SearchResponse ThisMessage {
@@ -407,7 +411,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class ResultItem : pb::GeneratedMessage<ResultItem, ResultItem.Builder> {
         private ResultItem() { }
-        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 {
@@ -415,7 +419,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override ResultItem DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override ResultItem ThisMessage {
@@ -518,6 +522,10 @@ 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(); }
@@ -533,22 +541,22 @@ 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;
@@ -563,13 +571,13 @@ 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);
@@ -585,11 +593,11 @@ 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) {
@@ -797,6 +805,11 @@ 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(); }
@@ -812,22 +825,22 @@ 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;
@@ -842,13 +855,13 @@ 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);
@@ -864,12 +877,11 @@ 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) {
@@ -998,7 +1010,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class RefineSearchRequest : pb::GeneratedMessage<RefineSearchRequest, RefineSearchRequest.Builder> {
     private RefineSearchRequest() { }
-    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 {
@@ -1006,7 +1018,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override RefineSearchRequest DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override RefineSearchRequest ThisMessage {
@@ -1035,12 +1047,12 @@ 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 {
@@ -1117,6 +1129,11 @@ 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(); }
@@ -1132,22 +1149,22 @@ 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;
@@ -1162,13 +1179,13 @@ 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);
@@ -1184,12 +1201,11 @@ 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) {
@@ -1344,7 +1360,7 @@ 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

@@ -140,7 +140,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlChild : pb::GeneratedMessage<TestXmlChild, TestXmlChild.Builder> {
     private TestXmlChild() { }
-    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 {
@@ -148,7 +148,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlChild DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlChild ThisMessage {
@@ -259,6 +259,11 @@ 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(); }
@@ -275,22 +280,22 @@ 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;
@@ -305,13 +310,13 @@ 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);
@@ -327,12 +332,11 @@ 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) {
@@ -486,7 +490,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlNoFields : pb::GeneratedMessage<TestXmlNoFields, TestXmlNoFields.Builder> {
     private TestXmlNoFields() { }
-    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 {
@@ -494,7 +498,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlNoFields DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlNoFields ThisMessage {
@@ -564,6 +568,10 @@ 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(); }
@@ -580,22 +588,22 @@ 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;
@@ -610,13 +618,13 @@ 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);
@@ -632,11 +640,11 @@ 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) {
@@ -715,7 +723,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlRescursive : pb::GeneratedMessage<TestXmlRescursive, TestXmlRescursive.Builder> {
     private TestXmlRescursive() { }
-    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 {
@@ -723,7 +731,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlRescursive DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlRescursive ThisMessage {
@@ -740,12 +748,12 @@ 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 {
@@ -809,6 +817,10 @@ 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(); }
@@ -825,22 +837,22 @@ 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;
@@ -855,13 +867,13 @@ 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);
@@ -877,11 +889,11 @@ 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) {
@@ -997,7 +1009,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearChild() {
         PrepareBuilder();
         result.hasChild = false;
-        result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlRescursive.DefaultInstance;
+        result.child_ = null;
         return this;
       }
     }
@@ -1012,7 +1024,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlMessage : pb::ExtendableMessage<TestXmlMessage, TestXmlMessage.Builder> {
     private TestXmlMessage() { }
-    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 {
@@ -1020,7 +1032,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlMessage ThisMessage {
@@ -1046,7 +1058,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Children : pb::GeneratedMessage<Children, Children.Builder> {
         private Children() { }
-        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 {
@@ -1054,7 +1066,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Children DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Children ThisMessage {
@@ -1165,6 +1177,11 @@ 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(); }
@@ -1181,22 +1198,22 @@ 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;
@@ -1211,13 +1228,13 @@ 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);
@@ -1233,12 +1250,11 @@ 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) {
@@ -1445,12 +1461,12 @@ 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;
@@ -1576,6 +1592,13 @@ 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(); }
@@ -1592,22 +1615,22 @@ 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;
@@ -1622,13 +1645,13 @@ 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);
@@ -1644,14 +1667,11 @@ 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) {
@@ -1934,7 +1954,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearChild() {
         PrepareBuilder();
         result.hasChild = false;
-        result.child_ = global::Google.ProtocolBuffers.TestProtos.TestXmlChild.DefaultInstance;
+        result.child_ = null;
         return this;
       }
       
@@ -1993,7 +2013,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestXmlExtension : pb::GeneratedMessage<TestXmlExtension, TestXmlExtension.Builder> {
     private TestXmlExtension() { }
-    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 {
@@ -2001,7 +2021,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestXmlExtension DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestXmlExtension ThisMessage {
@@ -2088,6 +2108,10 @@ 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(); }
@@ -2104,22 +2128,22 @@ 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;
@@ -2134,13 +2158,13 @@ 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);
@@ -2156,11 +2180,11 @@ 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

@@ -127,7 +127,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpFileOptions : pb::GeneratedMessage<CSharpFileOptions, CSharpFileOptions.Builder> {
     private CSharpFileOptions() { }
-    private static readonly CSharpFileOptions defaultInstance = new Builder().BuildPartial();
+    private static readonly CSharpFileOptions defaultInstance = new CSharpFileOptions().MakeReadOnly();
     private static readonly string[] _cSharpFileOptionsFieldNames = new string[] { "add_serializable", "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[] { 72, 64, 48, 56, 1770, 1792, 32, 10, 40, 1786, 24, 1800, 18, 1778 };
     public static CSharpFileOptions DefaultInstance {
@@ -135,7 +135,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpFileOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpFileOptions ThisMessage {
@@ -429,6 +429,10 @@ 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(); }
@@ -444,22 +448,22 @@ 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;
@@ -474,13 +478,13 @@ 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);
@@ -496,11 +500,11 @@ 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) {
@@ -969,7 +973,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpFieldOptions : pb::GeneratedMessage<CSharpFieldOptions, CSharpFieldOptions.Builder> {
     private CSharpFieldOptions() { }
-    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 {
@@ -977,7 +981,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpFieldOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpFieldOptions ThisMessage {
@@ -1063,6 +1067,10 @@ 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(); }
@@ -1078,22 +1086,22 @@ 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;
@@ -1108,13 +1116,13 @@ 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);
@@ -1130,11 +1138,11 @@ 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) {
@@ -1240,7 +1248,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpServiceOptions : pb::GeneratedMessage<CSharpServiceOptions, CSharpServiceOptions.Builder> {
     private CSharpServiceOptions() { }
-    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 {
@@ -1248,7 +1256,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpServiceOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpServiceOptions ThisMessage {
@@ -1334,6 +1342,10 @@ 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(); }
@@ -1349,22 +1361,22 @@ 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;
@@ -1379,13 +1391,13 @@ 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);
@@ -1401,11 +1413,11 @@ 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) {
@@ -1511,7 +1523,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class CSharpMethodOptions : pb::GeneratedMessage<CSharpMethodOptions, CSharpMethodOptions.Builder> {
     private CSharpMethodOptions() { }
-    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 {
@@ -1519,7 +1531,7 @@ namespace Google.ProtocolBuffers.DescriptorProtos {
     }
     
     public override CSharpMethodOptions DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override CSharpMethodOptions ThisMessage {
@@ -1605,6 +1617,10 @@ 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(); }
@@ -1620,22 +1636,22 @@ 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;
@@ -1650,13 +1666,13 @@ 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);
@@ -1672,11 +1688,11 @@ 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

@@ -129,13 +129,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropPerson : pb::ExtendableMessage<TestInteropPerson, TestInteropPerson.Builder> {
     private TestInteropPerson() { }
-    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 {
@@ -168,13 +168,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class PhoneNumber : pb::GeneratedMessage<PhoneNumber, PhoneNumber.Builder> {
         private PhoneNumber() { }
-        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 {
@@ -239,6 +239,10 @@ 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(); }
@@ -254,22 +258,22 @@ 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;
@@ -284,13 +288,13 @@ 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);
@@ -306,11 +310,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override PhoneNumber BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -365,13 +369,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Addresses : pb::GeneratedMessage<Addresses, Addresses.Builder> {
         private Addresses() { }
-        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 {
@@ -467,6 +471,10 @@ 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(); }
@@ -482,22 +490,22 @@ 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;
@@ -512,13 +520,13 @@ 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);
@@ -534,11 +542,11 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Addresses BuildPartial() {
-            if (builderIsReadOnly) {
+            if (resultIsReadOnly) {
               return result;
             }
-            builderIsReadOnly = true;
-            return result;
+            resultIsReadOnly = true;
+            return result.MakeReadOnly();
           }
           
           
@@ -752,6 +760,13 @@ 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(); }
@@ -767,22 +782,22 @@ 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;
@@ -797,13 +812,13 @@ 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);
@@ -819,14 +834,11 @@ 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();
       }
       
       
@@ -1020,13 +1032,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropEmployeeId : pb::GeneratedMessage<TestInteropEmployeeId, TestInteropEmployeeId.Builder> {
     private TestInteropEmployeeId() { }
-    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 {
@@ -1081,6 +1093,10 @@ 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(); }
@@ -1096,22 +1112,22 @@ 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;
@@ -1126,13 +1142,13 @@ 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);
@@ -1148,11 +1164,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestInteropEmployeeId BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -1187,13 +1203,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMissingFieldsA : pb::GeneratedMessage<TestMissingFieldsA, TestMissingFieldsA.Builder> {
     private TestMissingFieldsA() { }
-    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 {
@@ -1218,13 +1234,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class SubA : pb::GeneratedMessage<SubA, SubA.Builder> {
         private SubA() { }
-        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 {
@@ -1291,6 +1307,11 @@ 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(); }
@@ -1306,22 +1327,22 @@ 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;
@@ -1336,13 +1357,13 @@ 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);
@@ -1358,12 +1379,11 @@ 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();
           }
           
           
@@ -1459,12 +1479,12 @@ 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) {
@@ -1497,6 +1517,10 @@ 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(); }
@@ -1512,22 +1536,22 @@ 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;
@@ -1542,13 +1566,13 @@ 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);
@@ -1564,11 +1588,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMissingFieldsA BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -1670,7 +1694,7 @@ 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;
       }
     }
@@ -1684,13 +1708,13 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestMissingFieldsB : pb::GeneratedMessage<TestMissingFieldsB, TestMissingFieldsB.Builder> {
     private TestMissingFieldsB() { }
-    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 {
@@ -1715,13 +1739,13 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class SubB : pb::GeneratedMessage<SubB, SubB.Builder> {
         private SubB() { }
-        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 {
@@ -1778,6 +1802,11 @@ 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(); }
@@ -1793,22 +1822,22 @@ 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;
@@ -1823,13 +1852,13 @@ 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);
@@ -1845,12 +1874,11 @@ 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();
           }
           
           
@@ -1926,12 +1954,12 @@ 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) {
@@ -1964,6 +1992,10 @@ 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(); }
@@ -1979,22 +2011,22 @@ 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;
@@ -2009,13 +2041,13 @@ 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);
@@ -2031,11 +2063,11 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override TestMissingFieldsB BuildPartial() {
-        if (builderIsReadOnly) {
+        if (resultIsReadOnly) {
           return result;
         }
-        builderIsReadOnly = true;
-        return result;
+        resultIsReadOnly = true;
+        return result.MakeReadOnly();
       }
       
       
@@ -2137,7 +2169,7 @@ 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

@@ -61,7 +61,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestRequiredLite : pb::GeneratedMessageLite<TestRequiredLite, TestRequiredLite.Builder> {
     private TestRequiredLite() { }
-    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 {
@@ -69,7 +69,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestRequiredLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestRequiredLite ThisMessage {
@@ -185,6 +185,10 @@ 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(); }
@@ -201,22 +205,22 @@ 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;
@@ -231,13 +235,13 @@ 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);
@@ -249,11 +253,11 @@ 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) {
@@ -376,7 +380,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropPersonLite : pb::ExtendableMessageLite<TestInteropPersonLite, TestInteropPersonLite.Builder> {
     private TestInteropPersonLite() { }
-    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 {
@@ -384,7 +388,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestInteropPersonLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestInteropPersonLite ThisMessage {
@@ -410,7 +414,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class PhoneNumber : pb::GeneratedMessageLite<PhoneNumber, PhoneNumber.Builder> {
         private PhoneNumber() { }
-        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 {
@@ -418,7 +422,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override PhoneNumber DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override PhoneNumber ThisMessage {
@@ -533,6 +537,10 @@ 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(); }
@@ -549,22 +557,22 @@ 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;
@@ -579,13 +587,13 @@ 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);
@@ -597,11 +605,11 @@ 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) {
@@ -725,7 +733,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
       public sealed partial class Addresses : pb::GeneratedMessageLite<Addresses, Addresses.Builder> {
         private Addresses() { }
-        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 {
@@ -733,7 +741,7 @@ namespace Google.ProtocolBuffers.TestProtos {
         }
         
         public override Addresses DefaultInstanceForType {
-          get { return defaultInstance; }
+          get { return DefaultInstance; }
         }
         
         protected override Addresses ThisMessage {
@@ -909,6 +917,10 @@ 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(); }
@@ -925,22 +937,22 @@ 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;
@@ -955,13 +967,13 @@ 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);
@@ -973,11 +985,11 @@ 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) {
@@ -1399,6 +1411,13 @@ 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(); }
@@ -1415,22 +1434,22 @@ 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;
@@ -1445,13 +1464,13 @@ 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);
@@ -1463,14 +1482,11 @@ 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) {
@@ -1759,7 +1775,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestInteropEmployeeIdLite : pb::GeneratedMessageLite<TestInteropEmployeeIdLite, TestInteropEmployeeIdLite.Builder> {
     private TestInteropEmployeeIdLite() { }
-    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 {
@@ -1767,7 +1783,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestInteropEmployeeIdLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestInteropEmployeeIdLite ThisMessage {
@@ -1863,6 +1879,10 @@ 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(); }
@@ -1879,22 +1899,22 @@ 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;
@@ -1909,13 +1929,13 @@ 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);
@@ -1927,11 +1947,11 @@ 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

@@ -44,7 +44,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessageLite : pb::GeneratedMessageLite<ImportMessageLite, ImportMessageLite.Builder> {
     private ImportMessageLite() { }
-    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 {
@@ -52,7 +52,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessageLite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessageLite ThisMessage {
@@ -147,6 +147,10 @@ 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(); }
@@ -162,22 +166,22 @@ 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;
@@ -192,13 +196,13 @@ 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);
@@ -210,11 +214,11 @@ 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

@@ -72,7 +72,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class ImportMessage : pb::GeneratedMessage<ImportMessage, ImportMessage.Builder> {
     private ImportMessage() { }
-    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 {
@@ -80,7 +80,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override ImportMessage DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override ImportMessage ThisMessage {
@@ -166,6 +166,10 @@ 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(); }
@@ -181,22 +185,22 @@ 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;
@@ -211,13 +215,13 @@ 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);
@@ -233,11 +237,11 @@ 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

@@ -33,7 +33,7 @@ namespace Google.ProtocolBuffers.TestProtos {
   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]
   public sealed partial class TestLiteImportsNonlite : pb::GeneratedMessageLite<TestLiteImportsNonlite, TestLiteImportsNonlite.Builder> {
     private TestLiteImportsNonlite() { }
-    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 {
@@ -41,7 +41,7 @@ namespace Google.ProtocolBuffers.TestProtos {
     }
     
     public override TestLiteImportsNonlite DefaultInstanceForType {
-      get { return defaultInstance; }
+      get { return DefaultInstance; }
     }
     
     protected override TestLiteImportsNonlite ThisMessage {
@@ -50,12 +50,12 @@ 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 {
@@ -136,6 +136,10 @@ 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(); }
@@ -151,22 +155,22 @@ 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;
@@ -181,13 +185,13 @@ 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);
@@ -199,11 +203,11 @@ 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) {
@@ -305,7 +309,7 @@ namespace Google.ProtocolBuffers.TestProtos {
       public Builder ClearMessage() {
         PrepareBuilder();
         result.hasMessage = false;
-        result.message_ = global::Google.ProtocolBuffers.TestProtos.TestAllTypes.DefaultInstance;
+        result.message_ = null;
         return this;
       }
     }