Regenerated code with changes
diff --git a/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs b/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
index 215b9b4..e6b39f4 100644
--- a/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
+++ b/src/ProtocolBuffers.Test/TestProtos/UnitTestCSharpOptionsProtoFile.cs
@@ -60,7 +60,7 @@
   [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]

   [global::System.CodeDom.Compiler.GeneratedCodeAttribute("ProtoGen", "2.3.0.277")]

   public sealed partial class OptionsMessage : pb::GeneratedMessage<OptionsMessage, OptionsMessage.Builder> {

-    private static readonly OptionsMessage defaultInstance = new Builder().BuildPartial();

+    private static readonly OptionsMessage defaultInstance = new OptionsMessage().MakeReadOnly();

     private static readonly string[] _optionsMessageFieldNames = new string[] { "customized", "normal", "options_message" };

     private static readonly uint[] _optionsMessageFieldTags = new uint[] { 26, 10, 18 };

     public static OptionsMessage DefaultInstance {

@@ -68,7 +68,7 @@
     }

     

     public override OptionsMessage DefaultInstanceForType {

-      get { return defaultInstance; }

+      get { return DefaultInstance; }

     }

     

     protected override OptionsMessage ThisMessage {

@@ -186,6 +186,10 @@
     public static OptionsMessage ParseFrom(pb::ICodedInputStream input, pb::ExtensionRegistry extensionRegistry) {

       return ((Builder) CreateBuilder().MergeFrom(input, extensionRegistry)).BuildParsed();

     }

+    private OptionsMessage MakeReadOnly() {

+      return this;

+    }

+    

     public static Builder CreateBuilder() { return new Builder(); }

     public override Builder ToBuilder() { return CreateBuilder(this); }

     public override Builder CreateBuilderForType() { return new Builder(); }

@@ -201,22 +205,22 @@
         get { return this; }

       }

       public Builder() {

-        result = DefaultInstance ?? new OptionsMessage();

-        builderIsReadOnly = result == DefaultInstance;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

       }

       internal Builder(OptionsMessage cloneFrom) {

         result = cloneFrom;

-        builderIsReadOnly = true;

+        resultIsReadOnly = true;

       }

       

-      bool builderIsReadOnly;

-      OptionsMessage result;

+      private bool resultIsReadOnly;

+      private OptionsMessage result;

       

       private OptionsMessage PrepareBuilder() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           OptionsMessage original = result;

           result = new OptionsMessage();

-          builderIsReadOnly = false;

+          resultIsReadOnly = false;

           MergeFrom(original);

         }

         return result;

@@ -231,13 +235,13 @@
       }

       

       public override Builder Clear() {

-        result = DefaultInstance ?? new OptionsMessage();

-        builderIsReadOnly = true;

+        result = DefaultInstance;

+        resultIsReadOnly = true;

         return this;

       }

       

       public override Builder Clone() {

-        if (builderIsReadOnly) {

+        if (resultIsReadOnly) {

           return new Builder(result);

         } else {

           return new Builder().MergeFrom(result);

@@ -253,11 +257,11 @@
       }

       

       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) {