C# SocketException was unhandled

Question

I just went through a YouTube tutorial on tcpip servers and clients and was using the code from it: https://www.youtube.com/watch?v=uXFso7xSSWk both part 1 and part 2. I have redone the code twice and have made sure that everything is the same. Whenever I run the server I get the error "SocketException was unhandled" at the line 'tcpListener.Start();' in the following method.

private void TcpServerRun()
        {
            TcpListener tcpListener = new TcpListener(IPAddress.Any, 5004);
            tcpListener.Start();
            updateUI("Listening");
            while (true)
            {
                TcpClient client = tcpListener.AcceptTcpClient();
                updateUI("Connected");
                Thread tcpHandlerThread = new Thread (new ParameterizedThreadStart(tcpHandler));
                tcpHandlerThread.Start(client);
            }
        }

I also get the same error when I run the client, but it is at the line 'client.Connect(IPAddress.Parse("10.3.29.252"), 5004);' in the following method:

private void ConnectAsClient()
        {
            TcpClient client = new TcpClient();
            client.Connect(IPAddress.Parse("10.3.29.252"), 5004);
            NetworkStream stream = client.GetStream();
            string s = "Hello from client";
            byte[] message = Encoding.ASCII.GetBytes(s);
            stream.Write(message, 0, message.Length);
            updateUI("Message send");
            stream.Close();
            client.Close();
        }

Any and all help is much appreciated. I am pretty new to coding and very new to c# so I am sorry for anything that maybe unclear.

Here is the entire code for the server:

namespace TcpServerTutorial
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void bStartServer_Click(object sender, EventArgs e)
        {
            Thread tcpServerRunThread = new Thread(new ThreadStart(TcpServerRun));
            tcpServerRunThread.Start();
            TcpServerRun();
        }

        private void TcpServerRun()
        {
            TcpListener tcpListener = new TcpListener(IPAddress.Any, 5004);
            tcpListener.Start();
            updateUI("Listening");
            TcpClient client = tcpListener.AcceptTcpClient();
            while (true)
            {
                TcpClient client = tcpListener.AcceptTcpClient();
                updateUI("Connected");
                Thread tcpHandlerThread = new Thread (new ParameterizedThreadStart(tcpHandler));
                tcpHandlerThread.Start(client);
            }


        }

        private void tcpHandler(object client)
        {
            TcpClient mClient = (TcpClient)client;
            NetworkStream stream = mClient.GetStream();
            byte[] message = new byte[1024];
            stream.Read(message, 0, message.Length);
            updateUI("New Message = " + Encoding.ASCII.GetString(message));
            stream.Close();
            mClient.Close();
        }

        private void updateUI(string s)
        {
            Func<int> del = delegate()
            {
                textBox1.AppendText(s + System.Environment.NewLine);
                return 0;
            };
            Invoke(del);
        }
    }
}

first thought is that address is not reachable. if you do a try catch and check the inner exception you might get more information. — hubson bropa
– hubson bropa, Commented Jun 15, 2015 at 14:56
SocketException.ErrorCode should give you a bit more specific detail on the kind of error you're getting. Any chance the dev machine you're using has a firewall blocking unexpected outbound connections? — David W
– David W, Commented Jun 15, 2015 at 14:59
when I put 'tcpListener.Start();' in a try catch it seems to pass that point, but then I get an "invalid operation exception" at 'TcpClient client = tcpListener.AcceptTcpClient();' — Gabriel Britcher
– Gabriel Britcher, Commented Jun 15, 2015 at 15:02
@DavidW I have tried turning off my firewall but it still gives the exception. — Gabriel Britcher
– Gabriel Britcher, Commented Jun 15, 2015 at 15:03
The additional information was: 'Only one usage of each socket address (protocol/network address/port) is normally permitted.' but I don't know what else could be running on that port or how to check. 5004 is usually free. — Gabriel Britcher
– Gabriel Britcher, Commented Jun 15, 2015 at 15:06

David W · Accepted Answer · 2015-06-15 15:47:25Z

1

Bingo.Look at this chunk of code:

    private void bStartServer_Click(object sender, EventArgs e)
    {
        // Called once when the thread starts
        Thread tcpServerRunThread = new Thread(new ThreadStart(TcpServerRun));
        tcpServerRunThread.Start();
        // Called again here
        TcpServerRun();
    }

The TCPServerRun() method is being called twice: once when your thread starts, and then again via the explicit call right after you start the thread (or even possibly in reverse order). That's the call where the SocketException occurs, because the server has already started. Remove the second reference and your problem should be resolved; look for a similar situation on your client.

answered Jun 15, 2015 at 15:47

David W

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1 Comment

David W Over a year ago

Awesome! Glad to have helped.

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