How do I cancel an asynchronous operation in Silverlight/WCF?

I am calling an asynchronous service from my Silverlight app and I want to be able to cancel that call after it is made. There is an option for e.Cancelled once the service has finished (i.e. If e.Cancelled Then), but how to you set that cancelled to true after you have called it? How do you cancel that asynchronous call?

Let me clarify a bit... what I am trying to do is call the SAME method twice, one right after the other, and get the results of the last call into my collection. If I call an asynchronous method twice there is no guarantee that the second call will return first, so I may end up with the results of the first call coming in last and having the wrong results in my collection. So what I would like to do is cancel the first call when I make the second so I don't get results back from the first call. Seeing as h开发者_C百科ow there is a Cancelled flag in the completed event args I figure you should be able to do this. But how?

---

It's async... the transfer is passed off to a remote server and it does not return until the server is done with it.

Basically the server will keep going, but you don't have to wait for the response. Disconnect your service completed event handler and pretend it was never called. That will give the effect of cancelling the operation.

If you really need to cancel something in progress on the server you would need to make another call to the server to cancel the first call. Assuming the first call is a very slow one, this might be possible.

Update (as question changed)

In the case you specify, it will be up to the server to cancel a operation in progress if a second one comes through, not up to the client. e.Cancelled is set server-side.

However... :)

You have exposed a client usability issue. Shouldn't you also delay sending any service request until an idle delay has passed. That way rapid selections will not result in multiple service calls.

Also... :>

You may also want to send a sequence number to your service calls and return that as part of the result. Then you will know if it is the latest request or not.

---

It sounds like what you really want to do is ignore the responses of all but the most recent call.

Set a unique ID (could be request #, a Guid, timestamp, or whatever) with the request, and make sure the service sends that same value back. Keep around the ID of the most recent request and ignore response that don't match that ID.

This will be safer than cancelling the first request, since if the service has already started sending the response before the cancel request happens, you still get your error condition.