Fully converted over to ICodedOutputStream for all WriteTo derivations
diff --git a/src/ProtocolBuffers/ExtendableMessage.cs b/src/ProtocolBuffers/ExtendableMessage.cs
index 7e988c4..b5fbac3 100644
--- a/src/ProtocolBuffers/ExtendableMessage.cs
+++ b/src/ProtocolBuffers/ExtendableMessage.cs
@@ -230,7 +230,7 @@
                 }

             }

 

-            public void WriteUntil(int end, CodedOutputStream output)

+            public void WriteUntil(int end, ICodedOutputStream output)

             {

                 while (next != null && next.Value.Key.FieldNumber < end)

                 {