c# - Sending and receiving UDP packets between two programs on the same computer

Question

Is it possible to get two separate programs to communicate on the same computer (one-way only) over UDP through localhost/127... by sharing the same port #?

We're working on a student project in which we need to send UDP packets containing some telemetry between two computers. The program that generates these packets is proprietary, but I'm working on the receiver program myself with C# using System.Net.Sockets.UdpClient and System.Net.IPEndPoint.

This works fine during our group's meetings when we have multiple computers connected on which we can run the two programs separately. But it's not very useful when I'm home and trying to expand on the telemetry processing program as I only have one computer (I need a feed for testing the processing program). I can not install the program on any of the school's computers either.

When I try to run both programs on my computer at the same time (starting my program last) I get a SocketException saying that only a single use of each port is normally allowed. Which leads me to believe there must be some way to share the port (although it makes sense that only a single program can use port on a computer at any one time, I have no trouble running multiple internet browsers at the same time (and I suppose they use port 80 for http)).

REEDIT of the EDIT:

sipwiz was right, and thanks to Kalmi for the pointer to UdpClient.Client.Bind(). At the time, though, we are considering using another program that generates similar packets, and with which we are able to share port with on the same computer using my first (although naive) approach with the UDP client binding in the ctor. Sorry for having to unmark your answer, sysrqb.

question from:https://stackoverflow.com/questions/687868/sending-and-receiving-udp-packets-between-two-programs-on-the-same-computer

Answer 1

I did not expect this to be possible, but.. well.. sipwiz was right.

It can be done very easily. (Please vote sipwiz's answer up!)

IPEndPoint localpt = new IPEndPoint(IPAddress.Any, 6000);

//Failed try
    try
    {
        var u = new UdpClient(5000);
        u.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);

        UdpClient u2 = new UdpClient(5000);//KABOOM
        u2.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
    }
    catch (Exception)
    {
        Console.WriteLine("ERROR! You must call Bind only after setting SocketOptionName.ReuseAddress. 
 And you must not pass any parameter to UdpClient's constructor or it will call Bind.");
    }

//This is how you do it (kudos to sipwiz)
    UdpClient udpServer = new UdpClient(localpt); //This is what the proprietary(see question) sender would do (nothing special) 

    //!!! The following 3 lines is what the poster needs...(and the definition of localpt (of course))
    UdpClient udpServer2 = new UdpClient();
    udpServer2.Client.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);
    udpServer2.Client.Bind(localpt);