How to fix following basic TCP IP Server code

Question

When server started by the button, below code gives an SystemNullReferenceException error in function named Runner. One of my friend says that the code is working in his computer, but I can not make it work in my computer. When I started the code by executable file, it falls into not responding state, and I need to close it from task manager by killing the program. From textboxes IP is read as 127.0.0.1 and the port is 13000. I appreciate your guidance.

namespace FakeDataTCP_Server_NameSpace
{
    public partial class FakeDataTCP_ServerForm : Form
    {
        public FakeDataTCP_ServerForm()
        {
            InitializeComponent();
        }

        private void buttonServerRUN_Click(object sender, EventArgs e)
        {
            textBoxSTATUS.Clear();
            IPAddress sunucuIP = IPAddress.Parse(comboBoxServerIP.Text);
            int sunucuPORT = Int32.Parse(textBoxServerPORT.Text);
            textBoxSTATUS.AppendText("sunucu IP: " + sunucuIP.ToString() + "\r\n");
            textBoxSTATUS.AppendText("PORT: " + sunucuPORT.ToString() + "\r\n");

            Socket sunucuSoketListener = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            IPEndPoint sunucuEndIP = new IPEndPoint(sunucuIP, sunucuPORT);

            sunucuSoketListener.Bind(sunucuEndIP);
            sunucuSoketListener.Listen(100);

            textBoxSTATUS.AppendText("Server Started (Listening) \r\n");
            Socket istemciSoketi = default(Socket);
            FakeDataTCP_ServerForm p = new FakeDataTCP_ServerForm();
            Thread sunucuThread = new Thread(new ThreadStart(() => p.Runner(istemciSoketi)));
            sunucuThread.Start();

            int istemciSAYISI = 0;
            while (true)
            {
                istemciSAYISI++;
                istemciSoketi = sunucuSoketListener.Accept();
                textBoxSTATUS.AppendText(istemciSAYISI + " clients connected \r\n");
            }
        }
        public void Runner(Socket istci)
        {
            try
            {
                while (true)
                {
                    byte[] msg = new byte[1024];
                    int ebat = istci.Receive(msg);
                    istci.Send(msg, 0, ebat, SocketFlags.None);
                }
            }
            catch
            {
                MessageBox.Show("failed");
            }
        }

    }
}

Does this answer your question? [What is a NullReferenceException, and how do I fix it?](https://stackoverflow.com/questions/4660142/what-is-a-nullreferenceexception-and-how-do-i-fix-it) — Fildor, May 10 '22 at 21:07

Joel Coehoorn · Answer 1 · 2022-05-10T21:08:35.607

1

gives an SystemNullReferenceException error in function named Runner.

Looking at this function, I don't see anything inside that could itself be null, except for the istci Socket passed as an argument.

Working backward from there I see the function is called like this:

 p.Runner(istemciSoketi)

And working still further backwards I see the istemciSoketi variable is defined like this:

Socket istemciSoketi = default(Socket);

Aha! default(Socket) is null, so istemciSoketi doesn't have a value yet! The Socket you are trying to use was never actually created. You do assign a value later on:

istemciSoketi = sunucuSoketListener.Accept();

... but this doesn't happen until after p.Runner() was called.

To fix the issue, move the code calling p.Runner() to just after the SocketListener accepts a connection.

edited May 10 '22 at 21:08

answered May 10 '22 at 21:03

Joel Coehoorn

399,467
113
570
794

Thanks for your comment. Null error is resolved. Now with netstat command, I can see that server starts listening. May I ask your comment about program's not responding state? Why everytime I need to kill the program? – Minik May 10 '22 at 21:31
There's nothing to ever break out of the `while (true)` loop. – Joel Coehoorn May 11 '22 at 13:56

How to fix following basic TCP IP Server code

1 Answers1