Support Compact Framework 3.5

src/ProtocolBuffers/CodedInputStream.cs

@@ -163,7 +163,7 @@ namespace Google.ProtocolBuffers {
     /// Read a double field from the stream.
     /// </summary>
     public double ReadDouble() {
-#if SILVERLIGHT2
+#if SILVERLIGHT2 || COMPACT_FRAMEWORK_35
       byte[] bytes = ReadRawBytes(8);
       return BitConverter.ToDouble(bytes, 0);
 #else

src/ProtocolBuffers/CodedOutputStream.cs

@@ -332,7 +332,7 @@ namespace Google.ProtocolBuffers {
     /// </summary>
     public void WriteDoubleNoTag(double value) {
       // TODO(jonskeet): Test this on different endiannesses
-#if SILVERLIGHT2
+#if SILVERLIGHT2 || COMPACT_FRAMEWORK_35
       byte[] bytes = BitConverter.GetBytes(value);
       WriteRawBytes(bytes, 0, 8);
 #else
@@ -1140,4 +1140,4 @@ namespace Google.ProtocolBuffers {
       }
     }
   }
-}
+}

src/ProtocolBuffers/FieldAccess/ReflectionUtil.cs

@@ -64,13 +64,12 @@ namespace Google.ProtocolBuffers.FieldAccess {
     public static Func<TSource, object> CreateUpcastDelegateImpl<TSource, TResult>(MethodInfo method) {
       // Convert the reflection call into an open delegate, i.e. instead of calling x.Method()
       // we'll call getter(x).
-      Func<TSource, TResult> getter = (Func<TSource, TResult>)Delegate.CreateDelegate(typeof(Func<TSource, TResult>), method);
+      Func<TSource, TResult> getter = (Func<TSource, TResult>)Delegate.CreateDelegate(typeof(Func<TSource, TResult>), null, method);
 
       // Implicit upcast to object (within the delegate)
       return delegate(TSource source) { return getter(source); };
     }
 
-
     /// <summary>
     /// Creates a delegate which will execute the given method after casting the parameter
     /// down from object to the required parameter type.
@@ -84,7 +83,7 @@ namespace Google.ProtocolBuffers.FieldAccess {
     public static Action<TSource, object> CreateDowncastDelegateImpl<TSource, TParam>(MethodInfo method) {
       // Convert the reflection call into an open delegate, i.e. instead of calling x.Method(y) we'll
       // call Method(x, y)
-      Action<TSource, TParam> call = (Action<TSource, TParam>) Delegate.CreateDelegate(typeof(Action<TSource, TParam>), method);
+      Action<TSource, TParam> call = (Action<TSource, TParam>) Delegate.CreateDelegate(typeof(Action<TSource, TParam>), null, method);
 
       return delegate(TSource source, object parameter) { call(source, (TParam)parameter); };
     }
@@ -103,7 +102,7 @@ namespace Google.ProtocolBuffers.FieldAccess {
       // Convert the reflection call into an open delegate, i.e. instead of calling x.Method(y) we'll
       // call Method(x, y)
       Func<TSource, TParam, TReturn> call = (Func<TSource, TParam, TReturn>)
-          Delegate.CreateDelegate(typeof(Func<TSource, TParam, TReturn>), method);
+          Delegate.CreateDelegate(typeof(Func<TSource, TParam, TReturn>), null, method);
 
       return delegate(TSource source, object parameter) { call(source, (TParam)parameter); };
     }
@@ -118,7 +117,7 @@ namespace Google.ProtocolBuffers.FieldAccess {
     }
 
     public static Func<IBuilder> CreateStaticUpcastDelegateImpl<T>(MethodInfo method) {
-      Func<T> call = (Func<T>)Delegate.CreateDelegate(typeof(Func<T>), method);
+      Func<T> call = (Func<T>)Delegate.CreateDelegate(typeof(Func<T>), null, method);
       return delegate { return (IBuilder)call(); };
     }
   }

src/ProtocolBuffers/FieldAccess/RepeatedMessageAccessor.cs

@@ -52,7 +52,7 @@ namespace Google.ProtocolBuffers.FieldAccess {
     private readonly Func<IBuilder> createBuilderDelegate;
 
     internal RepeatedMessageAccessor(string name) : base(name) {
-      MethodInfo createBuilderMethod = ClrType.GetMethod("CreateBuilder", Type.EmptyTypes);
+      MethodInfo createBuilderMethod = ClrType.GetMethod("CreateBuilder", EmptyTypes);
       if (createBuilderMethod == null) {
         throw new ArgumentException("No public static CreateBuilder method declared in " + ClrType.Name);
       }

src/ProtocolBuffers/FieldAccess/RepeatedPrimitiveAccessor.cs

@@ -35,7 +35,7 @@ using System.Reflection;
 
 namespace Google.ProtocolBuffers.FieldAccess {
   /// <summary>
-  /// Accesor for a repeated field of type int, ByteString etc.
+  /// Accessor for a repeated field of type int, ByteString etc.
   /// </summary>
   internal class RepeatedPrimitiveAccessor<TMessage, TBuilder> : IFieldAccessor<TMessage, TBuilder>
       where TMessage : IMessage<TMessage, TBuilder>
@@ -49,7 +49,9 @@ namespace Google.ProtocolBuffers.FieldAccess {
     private readonly Func<TMessage, int> countDelegate;
     private readonly MethodInfo getElementMethod;
     private readonly MethodInfo setElementMethod;
-    
+
+    // Replacement for Type.EmptyTypes which apparently isn't available on the compact framework
+    internal static readonly Type[] EmptyTypes = new Type[0];
 
     /// <summary>
     /// The CLR type of the field (int, the enum type, ByteString, the message etc).
@@ -64,7 +66,7 @@ namespace Google.ProtocolBuffers.FieldAccess {
       PropertyInfo messageProperty = typeof(TMessage).GetProperty(name + "List");
       PropertyInfo builderProperty = typeof(TBuilder).GetProperty(name + "List");
       PropertyInfo countProperty = typeof(TMessage).GetProperty(name + "Count");
-      MethodInfo clearMethod = typeof(TBuilder).GetMethod("Clear" + name, Type.EmptyTypes);
+      MethodInfo clearMethod = typeof(TBuilder).GetMethod("Clear" + name, EmptyTypes);
       getElementMethod = typeof(TMessage).GetMethod("Get" + name, new Type[] { typeof(int) });
       clrType = getElementMethod.ReturnType;
       MethodInfo addMethod = typeof(TBuilder).GetMethod("Add" + name, new Type[] { ClrType });
@@ -78,9 +80,9 @@ namespace Google.ProtocolBuffers.FieldAccess {
           || setElementMethod == null) {
         throw new ArgumentException("Not all required properties/methods available");
       }
-      clearDelegate = (Func<TBuilder, IBuilder>)Delegate.CreateDelegate(typeof(Func<TBuilder, IBuilder>), clearMethod);
+      clearDelegate = (Func<TBuilder, IBuilder>)Delegate.CreateDelegate(typeof(Func<TBuilder, IBuilder>), null, clearMethod);
       countDelegate = (Func<TMessage, int>)Delegate.CreateDelegate
-          (typeof(Func<TMessage, int>), countProperty.GetGetMethod());
+          (typeof(Func<TMessage, int>), null, countProperty.GetGetMethod());
       getValueDelegate = ReflectionUtil.CreateUpcastDelegate<TMessage>(messageProperty.GetGetMethod());
       addValueDelegate = ReflectionUtil.CreateDowncastDelegateIgnoringReturn<TBuilder>(addMethod);
       getRepeatedWrapperDelegate = ReflectionUtil.CreateUpcastDelegate<TBuilder>(builderProperty.GetGetMethod());

src/ProtocolBuffers/FieldAccess/SingleMessageAccessor.cs

@@ -48,7 +48,7 @@ namespace Google.ProtocolBuffers.FieldAccess {
     private readonly Func<IBuilder> createBuilderDelegate;
 
     internal SingleMessageAccessor(string name) : base(name) {
-      MethodInfo createBuilderMethod = ClrType.GetMethod("CreateBuilder", Type.EmptyTypes);
+      MethodInfo createBuilderMethod = ClrType.GetMethod("CreateBuilder", EmptyTypes);
       if (createBuilderMethod == null) {
         throw new ArgumentException("No public static CreateBuilder method declared in " + ClrType.Name);
       }

src/ProtocolBuffers/FieldAccess/SinglePrimitiveAccessor.cs

@@ -46,6 +46,8 @@ namespace Google.ProtocolBuffers.FieldAccess {
     private readonly Func<TMessage, bool> hasDelegate;
     private readonly Func<TBuilder, IBuilder> clearDelegate;
 
+    internal static readonly Type[] EmptyTypes = new Type[0];
+
     /// <summary>
     /// The CLR type of the field (int, the enum type, ByteString, the message etc).
     /// As declared by the property.
@@ -59,13 +61,13 @@ namespace Google.ProtocolBuffers.FieldAccess {
       PropertyInfo builderProperty = typeof(TBuilder).GetProperty(name);
       if (builderProperty == null) builderProperty = typeof(TBuilder).GetProperty(name);
       PropertyInfo hasProperty = typeof(TMessage).GetProperty("Has" + name);
-      MethodInfo clearMethod = typeof(TBuilder).GetMethod("Clear" + name, Type.EmptyTypes);
+      MethodInfo clearMethod = typeof(TBuilder).GetMethod("Clear" + name, EmptyTypes);
       if (messageProperty == null || builderProperty == null || hasProperty == null || clearMethod == null) {
         throw new ArgumentException("Not all required properties/methods available");
       }
       clrType = messageProperty.PropertyType;
-      hasDelegate = (Func<TMessage, bool>)Delegate.CreateDelegate(typeof(Func<TMessage, bool>), hasProperty.GetGetMethod());
-      clearDelegate = (Func<TBuilder, IBuilder>)Delegate.CreateDelegate(typeof(Func<TBuilder, IBuilder>), clearMethod);
+      hasDelegate = (Func<TMessage, bool>)Delegate.CreateDelegate(typeof(Func<TMessage, bool>), null, hasProperty.GetGetMethod());
+      clearDelegate = (Func<TBuilder, IBuilder>)Delegate.CreateDelegate(typeof(Func<TBuilder, IBuilder>), null ,clearMethod);
       getValueDelegate = ReflectionUtil.CreateUpcastDelegate<TMessage>(messageProperty.GetGetMethod());
       setValueDelegate = ReflectionUtil.CreateDowncastDelegate<TBuilder>(builderProperty.GetSetMethod());
     }

src/ProtocolBuffers/FieldSet.cs

@@ -65,8 +65,8 @@ namespace Google.ProtocolBuffers {
     }
 
     public static FieldSet CreateInstance() {
-      // Use SortedDictionary to keep fields in the canonical order
-      return new FieldSet(new SortedDictionary<FieldDescriptor, object>());
+      // Use SortedList to keep fields in the canonical order
+      return new FieldSet(new SortedList<FieldDescriptor, object>());
     }
 
     /// <summary>
@@ -85,7 +85,7 @@ namespace Google.ProtocolBuffers {
       }
 
       if (hasRepeats) {
-        var tmp = new SortedDictionary<FieldDescriptor, object>();
+        var tmp = new SortedList<FieldDescriptor, object>();
         foreach (KeyValuePair<FieldDescriptor, object> entry in fields) {
           IList<object> list = entry.Value as IList<object>;
           tmp[entry.Key] = list == null ? entry.Value : Lists.AsReadOnly(list);

src/ProtocolBuffers/GeneratedMessage.cs

@@ -69,8 +69,8 @@ namespace Google.ProtocolBuffers {
 
     internal IDictionary<FieldDescriptor, Object> GetMutableFieldMap() {
 
-      // Use a SortedDictionary so we'll end up serializing fields in order
-      var ret = new SortedDictionary<FieldDescriptor, object>();
+      // Use a SortedList so we'll end up serializing fields in order
+      var ret = new SortedList<FieldDescriptor, object>();
       MessageDescriptor descriptor = DescriptorForType;
       foreach (FieldDescriptor field in descriptor.Fields) {
         IFieldAccessor<TMessage, TBuilder> accessor = InternalFieldAccessors[field];

src/ProtocolBuffers/MessageStreamIterator.cs

@@ -54,6 +54,9 @@ namespace Google.ProtocolBuffers {
     private readonly ExtensionRegistry extensionRegistry;
     private readonly int sizeLimit;
 
+    // Type.EmptyTypes isn't present on the compact framework
+    private static readonly Type[] EmptyTypes = new Type[0];
+
     /// <summary>
     /// Delegate created via reflection trickery (once per type) to create a builder
     /// and read a message from a CodedInputStream with it. Note that unlike in Java,
@@ -77,7 +80,7 @@ namespace Google.ProtocolBuffers {
         Type builderType = FindBuilderType();
 
         // Yes, it's redundant to find this again, but it's only the once...
-        MethodInfo createBuilderMethod = typeof(TMessage).GetMethod("CreateBuilder", Type.EmptyTypes);
+        MethodInfo createBuilderMethod = typeof(TMessage).GetMethod("CreateBuilder", EmptyTypes);
         Delegate builderBuilder = Delegate.CreateDelegate(
             typeof(Func<>).MakeGenericType(builderType), null, createBuilderMethod);
 
@@ -102,7 +105,7 @@ namespace Google.ProtocolBuffers {
     /// Works out the builder type for TMessage, or throws an ArgumentException to explain why it can't.
     /// </summary>
     private static Type FindBuilderType() {
-      MethodInfo createBuilderMethod = typeof(TMessage).GetMethod("CreateBuilder", Type.EmptyTypes);
+      MethodInfo createBuilderMethod = typeof(TMessage).GetMethod("CreateBuilder", EmptyTypes);
       if (createBuilderMethod == null) {
         throw new ArgumentException("Message type " + typeof(TMessage).FullName + " has no CreateBuilder method.");
       }

src/ProtocolBuffers/Properties/AssemblyInfo.cs

@@ -64,7 +64,9 @@ using System.Runtime.CompilerServices;
 // by using the '*' as shown below:
 // [assembly: AssemblyVersion("1.0.*")]
 [assembly: AssemblyVersion("1.0.0.0")]
+#if !COMPACT_FRAMEWORK_35
 [assembly: AssemblyFileVersion("1.0.0.0")]
+#endif
 
 [assembly:InternalsVisibleTo("Google.ProtocolBuffers.Test,PublicKey="+
 "00240000048000009400000006020000002400005253413100040000010001008179f2dd31a648"+

src/ProtocolBuffers/ProtocolBuffers.csproj

@@ -114,7 +114,7 @@
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="RpcUtil.cs" />
     <Compile Include="SilverlightCompatibility.cs" />
-    <Compile Include="SortedDictionary.cs" />
+    <Compile Include="SortedList.cs" />
     <Compile Include="TextFormat.cs" />
     <Compile Include="TextGenerator.cs" />
     <Compile Include="TextTokenizer.cs" />

src/ProtocolBuffers/ProtocolBuffersCF.csproj

@@ -0,0 +1,143 @@
+<Project DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="3.5">
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProductVersion>9.0.30729</ProductVersion>
+    <SchemaVersion>2.0</SchemaVersion>
+    <ProjectGuid>{49B51802-9D09-4AD8-A0EE-0DA704EFA379}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>Google.ProtocolBuffers</RootNamespace>
+    <AssemblyName>Google.ProtocolBuffers</AssemblyName>
+    <ProjectTypeGuids>{4D628B5B-2FBC-4AA6-8C16-197242AEB884};{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}</ProjectTypeGuids>
+    <PlatformFamilyName>PocketPC</PlatformFamilyName>
+    <PlatformID>b2c48bd2-963d-4549-9169-1fa021dce484</PlatformID>
+    <OSVersion>5.2</OSVersion>
+    <DeployDirSuffix>compactframework</DeployDirSuffix>
+    <TargetFrameworkVersion>v3.5</TargetFrameworkVersion>
+    <NativePlatformName>Windows Mobile 6 Professional SDK</NativePlatformName>
+    <FormFactorID>
+    </FormFactorID>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>COMPACT_FRAMEWORK_35;DEBUG;TRACE;$(PlatformFamilyName)</DefineConstants>
+    <NoStdLib>true</NoStdLib>
+    <NoConfig>true</NoConfig>
+    <ErrorReport>prompt</ErrorReport>
+    <FileAlignment>512</FileAlignment>
+    <WarningLevel>4</WarningLevel>
+    <GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>COMPACT_FRAMEWORK_35;TRACE;$(PlatformFamilyName)</DefineConstants>
+    <NoStdLib>true</NoStdLib>
+    <NoConfig>true</NoConfig>
+    <ErrorReport>prompt</ErrorReport>
+    <FileAlignment>512</FileAlignment>
+    <WarningLevel>4</WarningLevel>
+    <GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="mscorlib" />
+    <Reference Include="System" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="AbstractBuilder.cs" />
+    <Compile Include="AbstractMessage.cs" />
+    <Compile Include="ByteString.cs" />
+    <Compile Include="CodedInputStream.cs" />
+    <Compile Include="CodedOutputStream.cs" />
+    <Compile Include="Collections\Dictionaries.cs" />
+    <Compile Include="Collections\Enumerables.cs" />
+    <Compile Include="Collections\IPopsicleList.cs" />
+    <Compile Include="Collections\Lists.cs" />
+    <Compile Include="Collections\PopsicleList.cs" />
+    <Compile Include="Collections\ReadOnlyDictionary.cs" />
+    <Compile Include="Delegates.cs" />
+    <Compile Include="DescriptorProtos\CSharpOptions.cs" />
+    <Compile Include="DescriptorProtos\DescriptorProtoFile.cs" />
+    <Compile Include="DescriptorProtos\IDescriptorProto.cs" />
+    <Compile Include="DescriptorProtos\PartialClasses.cs" />
+    <Compile Include="Descriptors\DescriptorBase.cs" />
+    <Compile Include="Descriptors\DescriptorPool.cs" />
+    <Compile Include="Descriptors\DescriptorUtil.cs" />
+    <Compile Include="Descriptors\DescriptorValidationException.cs" />
+    <Compile Include="Descriptors\EnumDescriptor.cs" />
+    <Compile Include="Descriptors\EnumValueDescriptor.cs" />
+    <Compile Include="Descriptors\FieldDescriptor.cs" />
+    <Compile Include="Descriptors\FieldMappingAttribute.cs" />
+    <Compile Include="Descriptors\FieldType.cs" />
+    <Compile Include="Descriptors\FileDescriptor.cs" />
+    <Compile Include="Descriptors\IDescriptor.cs" />
+    <Compile Include="Descriptors\IndexedDescriptorBase.cs" />
+    <Compile Include="Descriptors\MappedType.cs" />
+    <Compile Include="Descriptors\MessageDescriptor.cs" />
+    <Compile Include="Descriptors\MethodDescriptor.cs" />
+    <Compile Include="Descriptors\PackageDescriptor.cs" />
+    <Compile Include="Descriptors\ServiceDescriptor.cs" />
+    <Compile Include="DynamicMessage.cs" />
+    <Compile Include="ExtendableBuilder.cs" />
+    <Compile Include="ExtendableMessage.cs" />
+    <Compile Include="ExtensionInfo.cs" />
+    <Compile Include="ExtensionRegistry.cs" />
+    <Compile Include="FieldAccess\FieldAccessorTable.cs" />
+    <Compile Include="FieldAccess\IFieldAccessor.cs" />
+    <Compile Include="FieldAccess\ReflectionUtil.cs" />
+    <Compile Include="FieldAccess\RepeatedEnumAccessor.cs" />
+    <Compile Include="FieldAccess\RepeatedMessageAccessor.cs" />
+    <Compile Include="FieldAccess\RepeatedPrimitiveAccessor.cs" />
+    <Compile Include="FieldAccess\SingleEnumAccessor.cs" />
+    <Compile Include="FieldAccess\SingleMessageAccessor.cs" />
+    <Compile Include="FieldAccess\SinglePrimitiveAccessor.cs" />
+    <Compile Include="FieldSet.cs" />
+    <Compile Include="GeneratedBuilder.cs" />
+    <Compile Include="GeneratedExtensionBase.cs" />
+    <Compile Include="GeneratedMessage.cs" />
+    <Compile Include="GeneratedRepeatExtension.cs" />
+    <Compile Include="GeneratedSingleExtension.cs" />
+    <Compile Include="IBuilder.cs" />
+    <Compile Include="IMessage.cs" />
+    <Compile Include="InvalidProtocolBufferException.cs" />
+    <Compile Include="IRpcChannel.cs" />
+    <Compile Include="IRpcController.cs" />
+    <Compile Include="IService.cs" />
+    <Compile Include="MessageStreamIterator.cs" />
+    <Compile Include="MessageStreamWriter.cs" />
+    <Compile Include="MessageUtil.cs" />
+    <Compile Include="NameHelpers.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="RpcUtil.cs" />
+    <Compile Include="SilverlightCompatibility.cs" />
+    <Compile Include="SortedList.cs" />
+    <Compile Include="TextFormat.cs" />
+    <Compile Include="TextGenerator.cs" />
+    <Compile Include="TextTokenizer.cs" />
+    <Compile Include="ThrowHelper.cs" />
+    <Compile Include="UninitializedMessageException.cs" />
+    <Compile Include="UnknownField.cs" />
+    <Compile Include="UnknownFieldSet.cs" />
+    <Compile Include="WireFormat.cs" />
+  </ItemGroup>
+  <Import Project="$(MSBuildBinPath)\Microsoft.CompactFramework.CSharp.targets" />
+  <ProjectExtensions>
+    <VisualStudio>
+      <FlavorProperties GUID="{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}">
+        <HostingProcess disable="1" />
+      </FlavorProperties>
+    </VisualStudio>
+  </ProjectExtensions>
+  <!-- To modify your build process, add your task inside one of the targets below and uncomment it. 
+       Other similar extension points exist, see Microsoft.Common.targets.
+  <Target Name="BeforeBuild">
+  </Target>
+  <Target Name="AfterBuild">
+  </Target>
+  -->
+</Project>

src/ProtocolBuffers/SortedDictionary.cs → src/ProtocolBuffers/SortedList.cs

@@ -46,7 +46,7 @@ namespace Google.ProtocolBuffers
     /// This is only used for Silverlight, which doesn't have the normal
     /// sorted collections.
     /// </summary>
-    internal sealed class SortedDictionary<TKey, TValue> : IDictionary<TKey, TValue>
+    internal sealed class SortedList<TKey, TValue> : IDictionary<TKey, TValue>
     {
         private readonly IDictionary<TKey, TValue> wrapped = new Dictionary<TKey, TValue>();
 

src/ProtocolBuffers/UnknownFieldSet.cs

@@ -243,11 +243,10 @@ namespace Google.ProtocolBuffers {
     public sealed class Builder
     {
       /// <summary>
-      /// Mapping from number to field. Note that by using a SortedDictionary we ensure
+      /// Mapping from number to field. Note that by using a SortedList we ensure
       /// that the fields will be serialized in ascending order.
       /// </summary>
-      private IDictionary<int, UnknownField> fields = new SortedDictionary<int, UnknownField>();
-
+      private IDictionary<int, UnknownField> fields = new SortedList<int, UnknownField>();
       // Optimization:  We keep around a builder for the last field that was
       // modified so that we can efficiently add to it multiple times in a
       // row (important when parsing an unknown repeated field).