Programming WCF Services, 3rd Edition

Callback Operations

WCF supports allowing a service to call back to its clients. During a callback, in many respects the tables are turned: the service is the client, and the client becomes the service (see Figure 5-1). Callback operations can be used in a variety of scenarios and applications, but they are especially useful when it comes to events, or notifying the client(s) that some event has happened on the service side.

Figure 5-1. A callback allows the service to call back to the client

Callbacks are also commonly referred to as duplex operations. There are two immediate challenges to supporting duplex communication. First, how does the service know where the callback endpoint is? Second, how does the client facilitate hosting the callback object?

Not all bindings support callback operations. Only bidirectional-capable bindings support callback operations. For example, because of its connectionless nature, HTTP cannot be used for callbacks, and therefore you cannot use callbacks over the BasicHttpBinding or the WSHttpBinding. The only two commonly used bindings that offer callbacks are the NetTcpBinding and the NetNamedPipeBinding, because by their very nature, the TCP and the IPC protocols support duplex communication.

The WSDualHttpBinding

To support callbacks over HTTP, WCF offers the WSDualHttpBinding, which actually sets up two WS channels: one for the calls from the client ...