I have this code...
internal static void Start()
{
TcpListener listenerSocket = new TcpListener(IPAddress.Any, 32599);
listenerSocket.Start();
listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
}
Then my call back function looks like this...
private static void AcceptClient(IAsyncResult asyncResult)
{
MessageHandler handler = new MessageHandler(listenerSocket.EndAcceptTcpClient(asyncResult));
ThreadPool.QueueUserWorkItem((object state) => handler.Process());
listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
}
Now, I call BeginAcceptTcpClient, then some time later I want to stop the server. To do this I have been calling TcpListener.Stop(), or TcpListener.Server.Close(). Both of these however execute my AcceptClient function. This then throws an exception when I call EndAcceptTcpClient. What is the best practice way around this? I could just put a flag in to stop the execution of AcceptClient once I have called stop, but I wonder if I am missing something.
Update 1
Currently I have patched it by changing the code to look like this.
private static void AcceptClient(IAsyncResult asyncResult)
{
if (!shutdown)
{
MessageHandler handler = new MessageHandler(listenerSocket.EndAcceptTcpClient(asyncResult));
ThreadPool.QueueUserWorkItem((object state) => handler.Process());
listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
}
}
private static bool shutdown = false;
internal static void Stop()
{
shutdown = true;
listenerSocket.Stop();
}
Update 2
I changed it to impliment the answer from Spencer Ruport.
private static void AcceptClient(IAsyncResult asyncResult)
{
if (listenerSocket.Server.IsBound)
{
MessageHandler handler = new MessageHandler(listenerSocket.EndAcceptTcpClient(asyncResult));
ThreadPool.QueueUserWorkItem((object state) => handler.Process());
listenerSocket.BeginAcceptTcpClient(new AsyncCallback(AcceptClient), null);
}
}
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 IsBound and the subsequent call to EndAcceptTcpClient(). You can still get an exception if the listener is Stop()'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, ObjectDisposedException (complaining that the underlying socket has already been disposed).
You should be able to check this by simulating the thread scheduling:
- Set a breakpoint on the line after the
IsBoundcheck in your callback - Freeze the thread that hits the breakpoint (Threads window -> right click, "Freeze")
- Run/trigger the code that calls
TcpListener.Stop() - Break in and step through the
EndAcceptTcpClient()call. You should see theObjectDisposedException.
IMO the ideal solution would be for Microsoft to throw a different exception from EndAcceptTcpClient in this case, e.g. ListenCanceledException or something like that.
As it is, we have to infer what's happening from the ObjectDisposedException. 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 TcpListener.Stop() in the first place). You should already have exception handling in that area anyway, since you can get various SocketExceptions. This is just tacking another catch handler onto that try block.
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 EndAcceptTcpClient() call that could otherwise trigger this exception. I hope.
Here's my code. This is early/prototype stuff, ignore the Console calls.
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);
}
No you're not missing anything. You can check the IsBound property of the Socket object. At least for TCP connections, while the socket is listening this will be set to true and after you call close it's value will be false. Though, your own implementation can work just as well.
try this one. it works fine for me without catching exceptions.
private void OnAccept(IAsyncResult pAsyncResult)
{
TcpListener listener = (TcpListener) pAsyncResult.AsyncState;
if(listener.Server == null)
{
//stop method was called
return;
}
...
}
i think that all tree things are needed and that the restart of BeginAcceptTcpClient should be placed outside the tryctach of EndAcceptTcpClient.
private void AcceptTcpClientCallback(IAsyncResult ar)
{
var listener = (TcpListener)ar.AsyncState;
//Sometimes the socket is null and somethimes the socket was set
if (listener.Server == null || !listener.Server.IsBound)
return;
TcpClient client = null;
try
{
client = listener.EndAcceptTcpClient(ar);
}
catch (SocketException ex)
{
//the client is corrupt
OnError(ex);
}
catch (ObjectDisposedException)
{
//Listener canceled
return;
}
//Get the next Client
listener.BeginAcceptTcpClient(new AsyncCallback(AcceptTcpClientCallback), listener);
if (client == null)
return; //Abort if there was an error with the client
MyConnection connection = null;
try
{
//Client-Protocoll init
connection = Connect(client.GetStream());
}
catch (Exception ex)
{
//The client is corrupt/invalid
OnError(ex);
client.Close();
}
}
This is a simple example how to start listening, how to process requests asynchronously, and how to stop listening.
Full example here.
public class TcpServer
{
#region Public.
// Create new instance of TcpServer.
public TcpServer(string ip, int port)
{
_listener = new TcpListener(IPAddress.Parse(ip), port);
}
// Starts receiving incoming requests.
public void Start()
{
_listener.Start();
_ct = _cts.Token;
_listener.BeginAcceptTcpClient(ProcessRequest, _listener);
}
// Stops receiving incoming requests.
public void Stop()
{
// If listening has been cancelled, simply go out from method.
if(_ct.IsCancellationRequested)
{
return;
}
// Cancels listening.
_cts.Cancel();
// Waits a little, to guarantee
// that all operation receive information about cancellation.
Thread.Sleep(100);
_listener.Stop();
}
#endregion
#region Private.
// Process single request.
private void ProcessRequest(IAsyncResult ar)
{
//Stop if operation was cancelled.
if(_ct.IsCancellationRequested)
{
return;
}
var listener = ar.AsyncState as TcpListener;
if(listener == null)
{
return;
}
// Check cancellation again. Stop if operation was cancelled.
if(_ct.IsCancellationRequested)
{
return;
}
// Starts waiting for the next request.
listener.BeginAcceptTcpClient(ProcessRequest, listener);
// Gets client and starts processing received request.
using(TcpClient client = listener.EndAcceptTcpClient(ar))
{
var rp = new RequestProcessor();
rp.Proccess(client);
}
}
#endregion
#region Fields.
private CancellationToken _ct;
private CancellationTokenSource _cts = new CancellationTokenSource();
private TcpListener _listener;
#endregion
}
来源：https://stackoverflow.com/questions/1173774/stopping-a-tcplistener-after-calling-beginaccepttcpclient