c# - How to cancel an async WCF call?

Question

I have been trying some time to find out how to implement WCF call cancellation based on the new .NET 4.5 CancellationToken mechanism. All the samples I found are not WCF based, and do not cross the service boundary.

My service :

[ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)]
public class MyService : IMyService
{   
    public void LongOperation()
    {
        // do stuff that takes ages...
        // please cancel me!
    }
}

My Client (Desktop App):

Using an auto generated proxy :

private async void DoLongRunningTaskAsync()
{
    var asyncTask = _myService.LongOperationAsync();
    await asyncTask;
}

How do I cancel this task? Please provide a concrete example applicable to WCF on .NET 4.5+

EDIT:

The answers below seem to indicate that its impossible for technical reasons. So, imagine, I make my service ContextMode=Session, then set a static variable (in a separate service call) called cancellationPending=true, my original call is still running at this stage, and it periodically checks that variable. Would I still not be able to cancel? would it still be impossible? if so, why?

Answer 1

In two words - this is impossible.

If you understand what WCF call is by its nature, it becomes obvious.

Looking on your example, what we see generally: Two separate machine A and B, connected via networks. On machine A you have some component which listen a socket, and when it gets a command it starts synchronous long computation RemoteCall().

On machine B you have a client which has two autogenerated methods in fact

BeginRemoteCall->IAsyncResult + EndRemoteCall(IAsyncResult).

New Task based version of asynchronous call (aka RemoteCallAsync()->Task) by the nature is absolutely the same, and you can always create your own task using TaskFactory.FromAsync(BeginRemoteCall,EndRemoteCall)->Task

When you call BeginRemoteCall on the client, you send the command to the second machine "please start computation", and then you add a callback, which will be called, when calculation is done and results came. Internally this async implementation uses I/O completion ports and allows your client to be non-blocking.

But there is no way to cancel an operation on the server side, if it is already started. You may not be interested in results, you may ignore your callback, but the computation on the server side will be finished anyway.

That's it