Is an Open Channel Needed for Duplex Communication?

Question

Hi,

Short Version: When I've created a Channel using ChannelFactory on a client which uses duplex communication, do I need to keep the channel open in order to receive the callback or can I call ChannelFactory.Close()?

Long Version: I'm developing my first WCF service and I've created my own ClientProxy Class, which implements and amalgamates a few different services into one. I use a ChannelFactory to create each channel, and my general reading on the net has indicated I should cache the ChannelFactory, but I should only open and close the actual channel when its needed.

So I call ChannelFactory.Open to open a channel and perform a duplex operation (a one-way operation which later calls a callback). Should I close this channel by calling ChannelFactory.Close after I've requested the operation, and if I do, will I still receive the callback?

Basic testing seems to indicate I will receive the callback if I close the connection however I just want to be sure. Also, is this method of caching the ChannelFactory correct?

Thanks

Answer 1

You should keep the client side proxy open while you wish to receive callbacks and when done you should close the channel.

Here's a quote from the great book Programming WCF Services by Juval Lowy (I suggest you to read the whole chapter about callbacks):

5.3.4. Callback Connection Management

The callback mechanism supplies nothing like a higher-level protocol for managing the connection between the service and the callback endpoint. It is up to the developer to come up with some application-level protocol or a consistent pattern for managing the life cycle of the connection. As mentioned previously, the service can only call back to the client if the client-side channel is still open, typically done by not closing the proxy. Keeping the proxy open will also prevent the callback object from being garbage-collected. If the service maintains a reference on a callback endpoint and the client-side proxy is closed or the client application itself is gone, when the service invokes the callback, it will get an ObjectDisposedException from the service channel. It is therefore preferable for the client to inform the service when it no longer wishes to receive callbacks or when the client application is shutting down. To that end, you can add an explicit Disconnect( ) method to the service contract. Since every method call carries with it the callback reference, in the Disconnect( ) method the service can remove the callback reference from its internal store.