How do I fix the error "Only one usage of each socket address (protocol/network address/port) is normally permitted"?

Question

I've done a lot of googling but not had much luck with my issues. I am new to network programming and trying to learn, I've attempted to set up a simple server & client that communicate (following an online tutorial located here -> http://tech.pro/tutorial/704/csharp-tutorial-simple-threaded-tcp-server)

The issue I'm having is that I keep getting the exception "Only one usage of each socket address (protocol/network address/port) is normally permitted" when trying to start the TcpListener on the server.

I've tried disabling my firewall, changing the port to be used, moving variables around but to no avail (the client works fine, but it obviously can't find the server because I cannot launch it).

I've seen solutions describing the use of Socket.Poll() but since I'm only using the TcpListener object, I have no idea how to make use of the Poll function.

My code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Sockets;
using System.Net;
using System.Threading;
using System.Text;

namespace ServerTutorial {
class Server {
    private readonly Thread m_listenThread;

    public Server() {
        m_listenThread = new Thread(new ThreadStart(ListenForClients));
        m_listenThread.Start();
    }

    public void ListenForClients() {
        var listener = new TcpListener(IPAddress.Any, 3000);
        listener.Start();

        while (true) {
            //Blocks until a client has connected to the server
            TcpClient client = listener.AcceptTcpClient();

            //Send a message to the client
            var encoder = new ASCIIEncoding();
            NetworkStream clientStream = client.GetStream();
            byte[] buffer = encoder.GetBytes("Hello Client!");
            clientStream.Write(buffer, 0, buffer.Length);
            clientStream.Flush();

            //Create a thread to handle communication with the connected client
            var clientThread = new Thread(new ParameterizedThreadStart(HandleClient));
            clientThread.Start(client);
        }
    }

    private void HandleClient(object clientObj) { //Param thread start can only accept object types, hence the cast
        var client = (TcpClient) clientObj;
        NetworkStream clientStream = client.GetStream();

        var message = new byte[4096];

        while (true) {
            int bytesRead = 0;

            try {
                //Block until a client sends a message
                bytesRead = clientStream.Read(message, 0, 4096);
            } catch {
                //A socket error has occurred
                System.Diagnostics.Debug.WriteLine("A socket error has occured");
                break;
            }

            if (bytesRead == 0) {
                //The client has disconnected from the server
                System.Diagnostics.Debug.WriteLine("A client has disconnected from the server");
                client.Close();
                break;
            }

            //Message has been received
            var encoder = new ASCIIEncoding();
            System.Diagnostics.Debug.WriteLine(encoder.GetString(message, 0, bytesRead));
        }
    }
}
}

In my main method:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ServerTutorial {
class Program {
    static void Main(string[] args) {
        var server = new Server();
        server.ListenForClients();
    }
}
}

Any help is hugely appreciated!

Answer 1

ListenForClients is getting invoked twice (on two different threads) - once from the constructor, once from the explicit method call in Main. When two instances of the TcpListener try to listen on the same port, you get that error.

Answer 2

You are debugging two or more times. so the application may run more at a time. Then only this issue will occur. You should close all debugging applications using task-manager, Then debug again.