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    <p>I just ran into this issue myself, and I believe your current solution is incomplete/incorrect. There is no guarantee of atomicity between the check for <code>IsBound</code> and the subsequent call to <code>EndAcceptTcpClient()</code>. You can still get an exception if the listener is <code>Stop()</code>'d between those two statements. You didn't say what exception you're getting but I assume it's the same one I'm getting, <code>ObjectDisposedException</code> (complaining that the underlying socket has already been disposed).</p> <p>You should be able to check this by simulating the thread scheduling:</p> <ul> <li>Set a breakpoint on the line after the <code>IsBound</code> check in your callback</li> <li>Freeze the thread that hits the breakpoint (Threads window -> right click, "Freeze")</li> <li>Run/trigger the code that calls <code>TcpListener.Stop()</code></li> <li>Break in and step through the <code>EndAcceptTcpClient()</code> call. You should see the <code>ObjectDisposedException</code>.</li> </ul> <p>IMO the ideal solution would be for Microsoft to throw a different exception from <code>EndAcceptTcpClient</code> in this case, e.g. <code>ListenCanceledException</code> or something like that.</p> <p>As it is, we have to infer what's happening from the <code>ObjectDisposedException</code>. Just catch the exception and behave accordingly. In my code I silently eat the exception, since I have code elsewhere that's doing the real shutdown work (i.e. the code that called <code>TcpListener.Stop()</code> in the first place). You should already have exception handling in that area anyway, since you can get various <code>SocketExceptions</code>. This is just tacking another catch handler onto that try block.</p> <p>I admit I'm uncomfortable with this approach since in principle the catch could be a false positive, with a genuine "bad" object access in there. But on the other hand there aren't too many object accesses in the <code>EndAcceptTcpClient()</code> call that could otherwise trigger this exception. I hope.</p> <p>Here's my code. This is early/prototype stuff, ignore the Console calls.</p> <pre><code> private void OnAccept(IAsyncResult iar) { TcpListener l = (TcpListener) iar.AsyncState; TcpClient c; try { c = l.EndAcceptTcpClient(iar); // keep listening l.BeginAcceptTcpClient(new AsyncCallback(OnAccept), l); } catch (SocketException ex) { Console.WriteLine("Error accepting TCP connection: {0}", ex.Message); // unrecoverable _doneEvent.Set(); return; } catch (ObjectDisposedException) { // The listener was Stop()'d, disposing the underlying socket and // triggering the completion of the callback. We're already exiting, // so just return. Console.WriteLine("Listen canceled."); return; } // meanwhile... SslStream s = new SslStream(c.GetStream()); Console.WriteLine("Authenticating..."); s.BeginAuthenticateAsServer(_cert, new AsyncCallback(OnAuthenticate), s); } </code></pre>
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    1. USDavid Pope
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    1. COYeah I ended up putting a stop bool that I set just before I call close. Then I check this before calling end accept.
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      1. USAnthony D
    2. COEven that approach suffers from the threading problem. Basically, unless there's a lock that covers both the boolean and calls to the TcpListener, you can get thread scheduling that causes the exception: 1) callback checks the boolean, we're still listening, cool, 2) that thread is swapped out for the thread that sets the boolean and calls Stop(), 3) the callback thread resumes and calls EndAccept.
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    3. COWhy do you cause the `ObjectDisposedException` by calling `EndAcceptTcpClient` on a closed listener instead of just checking whether it’s still live first, and simply returning from the callback?
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      1. USRoman Starkov
 

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