Added convenience methods for to/from xml and json
diff --git a/src/ProtocolBuffers/ExtendableBuilder.cs b/src/ProtocolBuffers/ExtendableBuilder.cs
index cbe203d..137762a 100644
--- a/src/ProtocolBuffers/ExtendableBuilder.cs
+++ b/src/ProtocolBuffers/ExtendableBuilder.cs
@@ -42,7 +42,7 @@
{
public abstract class ExtendableBuilder<TMessage, TBuilder> : GeneratedBuilder<TMessage, TBuilder>
where TMessage : ExtendableMessage<TMessage, TBuilder>
- where TBuilder : GeneratedBuilder<TMessage, TBuilder>
+ where TBuilder : GeneratedBuilder<TMessage, TBuilder>, new()
{
protected ExtendableBuilder()
{