C＃应用程序未在UDPClient.Receive上接收数据包

Question

I've come across a curious issue I can't seem to debug. My application received packets from a device sending UDP packets over a specific port. After setting up the UDP listener, a while loop triggers the Receive command periodically.

I should be receiving 400 values at every given time interval and I've even set a process to make sure these values are coming through. Below is a snippet of the relevant code:

public UdpClient listener;

IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, listenPort); 
//where listenPort is an int holding the port values should be received from

listener.ExclusiveAddressUse = false;
listener.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
listener.Client.Bind(groupEP);

if (listener.Client.Connected)
{
     listener.Connect(groupEP);
}
//I've actually never seen the app actually enter the code contained above

try
{
    while (!done)
    {
        if (isListenerClosed == false && currentDevice.isConnected)
        {
             try
             {
                  receive_byte_array = listener.Receive(ref groupEP); 
             }
             catch (Exception ex)
             {
                  throw ex;
             }
        }
    }
}
catch (SocketException ex)
{
    throw ex;
}

The odd thing is that the application runs just fine on my PC (both through setup file/Installshield and when run in Visual Studio) but won't receive any data when running off the setup file on a colleague's computer (it runs just fine in his Visual Studio environment). I've also tried attaching Visual Studio to the app's process, where I found that the code runs fine until it reaches listener.Receive . No exceptions are caught, no errors given in VS, but the code simply stops since no data is received.

Incidentally, both machines are identical (Mac Minis running 64-bit Windows 7 Ultimate N).

I've even included an UnhandledExceptionHandler in the Main Program as follows:

AppDomain.CurrentDomain.UnhandledException += new UnhandledExceptionEventHandler(CurrentDomain_UnhandledException);

private static void CurrentDomain_UnhandledException(object sender, UnhandledExceptionEventArgs e)
{
    MessageBox.Show("Unhandled Exception Caught " + e.ToString());
    throw new NotImplementedException();
}

Could this be an issue with Application permissions in Windows? Any ideas on the best approach to pinpointing the issue?

Answer 1

UDP is a connection-less protocol. Don't Connect . Instead, you're simply passing packets of data. Also, when you're using UdpClient , don't dig down to the underlying socket. There's no point.

The simplest (and quite stupid) UDP listener would look something like this:

var listener = new UdpClient(54323, AddressFamily.InterNetwork);

var ep = default(IPEndPoint);

while (!done)
{
    var data = listener.Receive(ref ep);

    // Process the data
}

Doing all the stuff around ExclusiveAddressUse (and SocketOptionName.ReuseAddress ) only serves to hide problems from you. Unless you're using broadcast or multi-cast, only one of the UDP listeners on that port will get the message. That's usually a bad thing.

If this simple code doesn't work, check the piping. Firewalls, IP addresses, drivers, the like. Install WireShark and check that the UDP packets are actually coming through - it might be the device's fault, it might be wrong configuration.

Also, ideally you'd want to do all this asynchronously. If you've got .NET 4.5, this is actually quite easy.

Answer 2

If you are running this on Windows Vista or beyond , it is probably the UAC . It can quietly prevent sockets from working properly. If you turn the UAC level down, it won't just block the socket.