Server‑Side Port Listening

Build the server‑side program, bind an IP address and port via TcpListener, start port listening and wait for incoming TCP connections from clients. Two implementation versions are provided: Console Application and WinForms Window Application.

Core API Reference

1. IPAddress: Represents the local machine IP address. IPAddress.Parse("127.0.0.1") converts an IP string into an object.
   • 127.0.0.1: Local loopback address, accessible only from the local host. To allow connections from other devices on the LAN, use your local network IP such as 192.168.x.x;
   • IPAddress.Any: Listens on all network interface IP addresses of the host machine.
2. TcpListener: TCP server listener class. Constructor parameters: IP Address + Port Number
3. listener.Start(): Start listening. Once called, the server begins waiting for client connections.

Console Version

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

namespace TalkService
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create IP object
            IPAddress ip = IPAddress.Parse("127.0.0.1");
            // Create TCP listener listening on port 9500
            TcpListener listener = new TcpListener(ip, 9500);
            // Begin listening
            listener.Start();
            Console.WriteLine("Listening started... Press E to exit");

            ConsoleKeyInfo info;
            do
            {
                info = Console.ReadKey();
            } while (info.Key != ConsoleKey.E);

            listener.Stop(); // Stop listening on exit
        }
    }
}Code language: JavaScript (javascript)

This code only starts the listener. AcceptTcpClient has not been called to wait for client connections; it merely keeps the process running without terminating.

WinForms Version

UI Controls: (You can add controls to your WinForms form for setting IP, port, outgoing text content and more)

• textBoxIP: IP input box (default 127.0.0.1)
• textBoxPort: Port input box (default 9500)
• buttonStart: Start button
• textBoxInfo: Information display textbox

using System;
using System.Net;
using System.Net.Sockets;
using System.Windows.Forms;

namespace SocketChatServer
{
    public partial class Form1 : Form
    {
        IPAddress ip;
        TcpListener listener;

        public Form1()
        {
            InitializeComponent();
        }

        private void buttonStart_Click(object sender, EventArgs e)
        {
            // Parse IP and port
            ip = IPAddress.Parse(this.textBoxIP.Text);
            int port = Convert.ToInt32(this.textBoxPort.Text);
            listener = new TcpListener(ip, port);

            // Start listening
            listener.Start();

            // Output log message
            string log = $"Server started {DateTime.Now.ToShortTimeString()}\r\n";
            this.textBoxInfo.Text = log + this.textBoxInfo.Text;
        }
    }
}Code language: PHP (php)

Outstanding Issues

1. UI Freeze Pitfall
   listener.AcceptTcpClient() is a blocking method. Calling it directly from the main UI thread inside a button click handler will lock up your window. You need to spawn a new thread or use async/await to await incoming client connections asynchronously.
2. Port Occupation
   If port 9500 is occupied by another program, constructing new TcpListener() will throw an exception. Wrap this section with try‑catch during development for error handling.
3. Cross‑host Connectivity
   Binding to 127.0.0.1 works only for local testing. To accept connections from other LAN devices, set the IP to your machine’s local network address and allow port 9500 through your firewall.

Server‑Side Port Listening