private static void serializeCallbackResult(JsonWriter writer, CallbackResult result)
        {
            if (result == null)
            {
                writer.Write(null);
            }
            else
            {
                writer.WriteObjectStart();
                writer.WritePropertyName("type");

                if (result is FlowCallbackResult)
                    writer.Write("flow");
                else if (result is MessageCallbackResult)
                        writer.Write("message");
                    else if (result is FormCallbackResult)
                            writer.Write("form");
                writer.WritePropertyName("value");
                result.Write(writer, false);
                writer.WriteObjectEnd();
            }
        }