Duplex Service in WCF（CodeProject上的）
In WCF, a service can call back to its clients.
That is to say that, at the time of call back, the service behaves as a client as well as the client becomes the service, and the client must assist hosting the call back object.
To support call back, the underlying transport of the binding must support bidirectional.
So, for the nature of connectionless, we cannot use all bindings for the call back operation.
For instance, BasicHttpBinding or WsHttpBinding does not support callback while WsDualHttpBinding supports it, because this binding establishes two HTTP channels to do that; one for client to service another for service to client.
Also, NetTcpBinding and NetNamedPipeBinding supports callback since their underlying transport is bidirectional.
Callback service can be supported by using CallbackContract property in the ServiceContract attribute.
Only one callback contract is allowed for a service contract.
After defining a callback contract, we require to set up the client for up keeping the callback and also to offer the callback endpoint to the service for each and every call.
In the example given below, the callback contract defines the operations to which the service can call on the client endpoint.
