How can I make every instance of the program use a different port for p2p

Question

I am making a p2p program in c# and I would like to open up multiple instances of the program on my pc and connect them p2p over sockets. Each one should use a different port (if possible incremented by one). How could I accomplish this? The only thing I can think of is randomly generating them, but even so there could be two same ports. Thank you

Answer 1

When opening a socket with a port that is already in use, you get a socket exception. You can catch that exception and increase the port number. E.g. like this:

using System.Net;
using System.Net.Sockets;

public class PortOpener
{
    private static Socket FindOpenPort(int startPort, int endport)
    {
        for (int port = startPort; port < endport; port++)
        {
            try
            {
                var socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
                socket.Bind(new IPEndPoint(IPAddress.Any, port));
                return socket;
            }
            catch (SocketException ex)
            {
                if (ex.SocketErrorCode != SocketError.AddressAlreadyInUse) throw;
            }
        }
        return null;
    }

    public static void Main()
    {
        var socket = FindOpenPort(8000, 8100);
    }
}