How do I cancel an asynchronous operation?

C#

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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,
Jacob



Re: How do I cancel an asynchronous operation?

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

Jacob,


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
 http://www.**--****.com/ 
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.

Jacob


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


not
.NET



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

        while (true) {
            allDone.Reset();

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

            allDone.WaitOne();
        }
    } catch (Exception e) {
        Console.WriteLine(e.ToString());
    }

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
done.

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!

// CHRIS


Similar Threads:

1.Cancel an Asynchronous operation?

I can start an Asynchronous operation against a data source with 
SQLCommand.BeginExecuteReader, allowing me to loop, checking for user 
cancellation before the operation has completed, but how then to cancel the 
SQLCommand if the user wishes to?  The "Cancel" method states:

     ".......... The Cancel method cannot be used to cancel a pending 
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The scenario I'm thinking about here concerns a potentially long-running 
operation the user might start not realising how long it might take, despite 
a UI warning, then deciding to cancel it.  Because I don't know if the 
operation will take seconds or minutes, I can't set an appropriate timeout 
other than "0", which means the user must be allowed to cancel it.  Is there 
any way to stop an asynchronous operation "mid-flow" so to speak, without 
running it on a thread and aborting the thread (not a very clever thing to 
do)?  What about closing the underlying connection or generating an 
exception, will that stop the pending operation?


Robin


2.Cancel an asynchronous method call made to delegate using BeginInv

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returned by the BeginInvoke.

I have tried using the Close method of the WaitHandle object to end the call.

While I'm asking, please explain (in more detail than the MSDN Library) what 
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-- 
Jeffrey Kingsley
Xerox Inovation Group
Xerox Corporation

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4.Newbie question on performing/handling Asynchronous operations..

Hi,

   I have a dotnet form that basically writes to the webserver when a
button is clicked. Now based on the amount of data, the writing could
take about 5-10 minutes.

   I was wondering if there was a way for me to post a message back to
the user
as soon as the button is clicked and then have the webserver continue
to perform the operation. This way the user gets an immediate repsonse
saying that teh operationis being performed and does'nt have to wait
that long. This will also take care of potential timeout situations.

   Can someone give me some examples/ code snippets/links that will
help me out
if it indeed is possible. Please rememberthat I am new at this, so the
more
info the better and the more code with explanation the better.


  Regards,

DrD

5.asynchronous socket operations - question

I have a question about C#. How can I stop asynchronous read/write operation
( BeginReceive() / BeginSend() ) if timeout occurs?
The Socket class doesn't make any cancel method available.
I used CancelIo(HANDLE hFile) method (declared in Winbase.h) in C++.

Thanks for help.
Olek


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