I got to use WCF callbacks recently and realized some of it's gotchas, which are listed here:

1. Callbacks requires communication over duplex channel
2. Requires all clients to implement the callback interface, exposed by the service
3. BasicHttpBinding & WSHttpBinding don't support callbacks -- use their Dual versions instead
4. Streaming data not supported by callbacks -- streaming is not reliable and Callbacks require reliable messaging
5. Requires reliable messaging
6. If the service restarts, callbacks are not possible until the client makes a service connection again & subscribes itself for callbacks.

If you'd like to know more about how to setup WCF callbacks etc use this link: http://msdn2.microsoft.com/en-us/magazine/cc163537.aspx

Enjoy!

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