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/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

 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/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/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) {