UDP - Sending a multicast message and listening for response gives SocketException

Question

I'm sending a UDP multicast message to find certain devices on the network. I then listen for a response on port 5001.

My workflow is as follows:

Send a multicast "find" message to the local subnet.
Any wiznet devices on the network respond with an info packet which I want to receive

It all works fine on Windows XP, but on Windows 7, I get an exception of type SocketException: *

Only one usage of each socket address (protocol/network address/port) is normally permitted

I can see the multicast message go out in Wireshark, and I see the response from the device(s), but my code doesn't respond. My code looks like this:

  public void StartListen()
  {
      SendFind();
      try {
          IPEndPoint localEp = new IPEndPoint(IPAddress.Any, 0);
          UdpClient listenClient = new UdpClient(5001);

          UdpState s = new UdpState();
          s.endpoint = localEp;
          s.client = listenClient;

          //allow time for the find to work - aka clutching at straws
          Thread.Sleep(500);

          while (listenClient.Available > 0)
          {
              listenClient.BeginReceive(ReceiveCallback, s);
              Thread.Sleep(500);
          }
      }

      catch (SocketException e)
      {
          Trace.WriteLine("Could not bind to socket on " + _localPort);
      }

      listenClient.Close();
  }

.. and RecieveCallBack ..

private void ReceiveCallback(IAsyncResult ar)
{
    UdpClient u = (UdpClient)((UdpState)(ar.AsyncState)).client;
    IPEndPoint e = (IPEndPoint)((UdpState)(ar.AsyncState)).endpoint;
    Byte[] receiveBytes = u.EndReceive(ar, ref e);
    rxByteList.Add(receiveBytes);
    messageRxed = true;
}

** UPDATE **

So I have tried various ways of structuring this code. It seems that the problem is related to sending and receiving on different UdpClients. My exception was caused by creating a UdpClient to recieve immediately after opening one to send - adding a delay between the send and receive fixed this.

I have altered my code to use the same UdpClient for the send and receive, but I'm still not getting anything on the receive.

Obviously there is already a socket bound to port 5001. This may be an application already running, or your own code. Double-check that you're not binding multiple sockets to the same port, and try if a different port works. — dtb
I may be missing something obvious, but I'm fairly certain that isn't the problem. I have nothing else bound to port 5001, and the code works on xp. Also have tried this on a "clean" windows 7 machine with no joy — Matt Roberts
Actually, that's not where the exception is thrown, but rather where the exception is caught! The exception is thrown before that, probably by the call to the UdpClient constructor, or perhaps by the call to BeginReceive. Can you tell us which? — Jim Mischel
On what machine is the program that sends the multicast running? — Jim Mischel
Sorry - stupid mistake with the exception comment! It is thrown in the call to the UdpClient constructor. The weird thing is that if I step through the program from the start and "F10" that constructor call, no exception is thrown... The multicast is sent from my machine (win 7) — Matt Roberts

Matt Roberts Matt Roberts · Accepted Answer · 2011-06-07T13:55:57

Sorted! It was because my Sendpacket used "udpclient.connect", which restricts the receive to all data in the endpoint supplied to the connect method.

UDP - Sending a multicast message and listening for response gives SocketException

1 Answers