Duplex Service in WCF .net Application:A duplex service contract is a message exchange pattern in which both endpoints can send messages to the other independently. A duplex service, therefore, can send messages back to the client endpoint, providing event-like behavior. Duplex communication occurs when a client connects to a service and provides the service with a channel on which the service can send messages back to the client. Note that the event-like behavior of duplex services only works within a session.
To create a duplex contract you create a pair of interfaces. The first is the service contract interface that describes the operations that a client can invoke. That service contract must specify a callback contract in the System.ServiceModel.ServiceContractAttribute.CallbackContract property. The callback contract is the interface that defines the operations that the service can call on the client endpoint. A duplex contract does not require a session, although the system-provided duplex bindings make use of them.