Question

I’m developing a server-client application that uses 3 ports [TCP SOCKET .Net 4.0]..

So the application gives the user the choice to set the port for the main socket only. but I want to let the server application to automatically find available port for the other 2 sockets so it sends the port value to the client using the main socket connection. then the client connect to the other socket using the received port value.

here’s a little explanation:

the main socket listens on a configurable port. this socket accepts client to start send/receive commands. (file explorer/task manager/properties/shutdown/chat)

the second socket is for file transfer only to transfer files and it closes when its done.

the third socket is only for receive/send a screenshot.

[i know you might say that i should use the same socket for file transfer and screenshot but its a little complicated. i just use separate sockets for each one of them.]

so how do i find an available port before bind the socket with the endpoint? something like this :

int port = 10000;
bool isAvailable = false;
while(!isAvailable)
{
    try
    { 
        // check if the port is available to use.
        isAvailable = true;
    }
    catch
    {
        port++;
    }
} 

Answer 1

If the port number doesn’t matter you could pass 0 for the port to the IPEndPoint. In this case
the operating system (TCP/IP stack) assigns a free port number for you.

Socket sock = new Socket(AddressFamily.InterNetwork,
                         SocketType.Stream, ProtocolType.Tcp);
sock.Bind(new IPEndPoint(IPAddress.Parse("192.168.0.1"), 0)); // Pass 0 here.

Console.Out.WriteLine("Assigned port: {0}",
                      ((IPEndPoint)sock.LocalEndPoint).Port);

As @GrokSrc pointed out in the comments section you should of course dispose the socket when you’re done with the socket by using the Dispose() method or the using statement.