Added option generate_private_ctor to disable private ctor generation

protos/google/protobuf/csharp_options.proto

@@ -42,6 +42,9 @@ message CSharpFileOptions {
   // Generate messages/builders with the [Serializable] attribute
   optional bool add_serializable = 9 [default = false];
   
+  // Generates a private ctor for Message types
+  optional bool generate_private_ctor = 10 [default = true];
+
   // The extension that should be appended to the umbrella_classname when creating files.
   optional string file_extension = 221 [default = ".cs"];
   

src/ProtoGen/MessageGenerator.cs

@@ -181,8 +181,11 @@ namespace Google.ProtocolBuffers.ProtoGen
                              Descriptor.Proto.ExtensionRangeCount > 0 ? "Extendable" : "Generated",
                              RuntimeSuffix);
             writer.Indent();
+            if (Descriptor.File.CSharpOptions.GeneratePrivateCtor)
+            {
                 writer.WriteLine("private {0}() {{ }}", ClassName);
-            // Must call BuildPartial() to make sure all lists are made read-only
+            }
+            // Must call MakeReadOnly() to make sure all lists are made read-only
             writer.WriteLine("private static readonly {0} defaultInstance = new {0}().MakeReadOnly();", ClassName);
 
             if (OptimizeSpeed)