How do I cancel an asynchronous operation?


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How do I cancel an asynchronous operation?

Postby Jacob » Sun, 28 Dec 2003 13:16:18 GMT

I'm writing a class that  communicates with a server using the TcpClient
class.  Most of the methods I've written are intended to be used
synchronously and will block until they are completed.  But these calls will
be made very seldom and between operations I would like to put the client
into a "listening mode" that will listen for other server messages.  When
the client is ready to send/receive data again, take it back out of
listening mode and again begin synchronous operations.  It's quite easy to
set up a BeginRead on the stream I created and have its callback process any
messages received from the server.  But when I want to STOP listening and
start synchronous operations again I need to be able to cancel this.  I
think the easiest way would be if I could just cancel the thread that I
started the BeginRead operation on.  I'm sure there is a way to use the
IAsyncResult or it's WaitHandle to cancel the thread, but threading is not
my strongpoint and a solution is alluding me right now.  Any assistance
would be appreciated.  Here's the skeleton of what I'm going for.

private IAsyncResult ar;

private void Listen()
    listenBuffer = new byte[1024];
    ar = stream.BeginRead(listenBuffer, 0, listenBuffer.Length, new
AsyncCallback(ListenMessage), this);

private void ListenMessage(IAsyncResult ar)
    // ..... interpret received message.

private void StopListen()
    // ?????? ar...

Any help is appreciated,

Re: How do I cancel an asynchronous operation?

Postby Dmitriy Lapshin [C# / .NET MVP] » Tue, 30 Dec 2003 21:36:35 GMT


Unfortunately, cancelling the async. operations is not supported by the .NET
framework. I have recently had to write my own replacement since
cancellation was a must.

Dmitriy Lapshin [C# / .NET MVP]
X-Unity Test Studio
Bring the power of unit testing to VS .NET IDE

Re: How do I cancel an asynchronous operation?

Postby Jacob » Wed, 31 Dec 2003 02:42:04 GMT

Well, that answers my question.  Thanks for your help.


"Dmitriy Lapshin [C# / .NET MVP]" < XXXX@XXXXX.COM > wrote


Re: How do I cancel an asynchronous operation?

Postby Chris Morse » Fri, 02 Jan 2004 14:21:58 GMT

On Mon, 29 Dec 2003 14:36:35 +0200, "Dmitriy Lapshin [C# / .NET MVP]"

I am just researching this same problem.  In my case, it's a server
socket that calls "BeginAccept()".  The MSDN doc sample "Using an
Asynchronous Server Socket" shows this code:

    try {
        s.Listen(10);			// <-- typo in MSDN docs!

        while (true) {

            Console.WriteLine("Waiting for a connection...");
                new AsyncCallback(SocketListener.acceptCallback), 
                listener );

    } catch (Exception e) {

An infinite loop of BeginAccept() calls is NOT what I would want in my
own server.  As I have been trying to figure out how to cancel a
"pending" BeginAccept() call, I am finding out now that it can't be

It's not too big a problem for me, as my own version has a way out of
the loop and closes the server socket.  Either the server socket
closing OR the thread ending (which happens after the close) causes
the acceptCallback to be called ONCE the first time, and if I continue
hitting my server start and stop buttons, the acceptCallback will be
called TWICE everytime after that.  Strange behaviour.

So I have been trying to figure out how to cancel the pending
BeginAccept() call.  And now it seems it can't be done!


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