public void Ping_Xml(Guid guid) { xml_callback = OperationContext.Current.GetCallbackChannel<IWcfDuplexService_Xml_Callback>(); XmlCompositeTypeDuplexCallbackOnly xmlCompositeType = new XmlCompositeTypeDuplexCallbackOnly(); xmlCompositeType.StringValue = guid.ToString(); // Schedule the callback on another thread to avoid reentrancy. Task.Run(() => xml_callback.OnXmlPingCallback(xmlCompositeType)); }