服务器端软件：

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
// 定义新的命名空间。
using System.Net;
using System.Net.Sockets;
using System.IO;
using System.Text;
using System.Threading;


namespace TalkServer
{
    /// <summary>
    /// 服务器端软件。
    /// </summary>
    public class Form1 : System.Windows.Forms.Form
    {
        private System.Windows.Forms.GroupBox groupBox1;
        private System.Windows.Forms.Panel panel1;
        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.RichTextBox richTextBox1;
        private System.Windows.Forms.RichTextBox richTextBox2;
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        private System.ComponentModel.Container components = null;

        public Form1()
        {
            //
            // Windows 窗体设计器支持所必需的
            //
            InitializeComponent();

            //
            // TODO: 在 InitializeComponent 调用后添加任何构造函数代码
            //
        }

        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null)
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        Windows Form Designer generated code

        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.Run(new Form1());
        }
        // 定义私有变量。
        public bool m_bConnected = false;
        public IPEndPoint p = null;
        public Socket s = null;
        public NetworkStream ns = null;
        public TextReader r = null;
        public TextWriter w = null;
        public Thread t = null;
        public void ThreadListen()
        {
            Socket recv = null;
            recv = s.Accept();
            if(recv != null)
                m_bConnected = true;
            ns = new NetworkStream(recv);
            r = new StreamReader(ns);
            w = new StreamWriter(ns);
            while(m_bConnected)
            {
                string tmp;
                try
                {
                    tmp = r.ReadLine();
                    if(tmp.Length != 0)
                    {
                        lock(this)
                        {
                            richTextBox1.Text = "他说："+tmp+"\n"+richTextBox1.Text;
                        }
                    }
                }
                catch
                {
                    MessageBox.Show("连接断开！！");
                }
            }
            recv.Shutdown(SocketShutdown.Both);
            recv.Close();
            s.Shutdown(SocketShutdown.Both);
            s.Close();
        }
        private void button1_Click(object sender, System.EventArgs e)
        {
            p = new IPEndPoint(IPAddress.Any, 34567);
            s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            s.Blocking = true;
            s.Bind(p);
            s.Listen(0);
            t = new Thread(new ThreadStart(this.ThreadListen));
            t.Start();
            button1.Enabled = false;
        }
        private void richTextBox2_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            byte[] data;
            if(e.KeyChar == (char)13)
            {
                if(m_bConnected)
                {
                    try
                    {
                        lock(this)
                        {
                            richTextBox1.Text = "我说："+richTextBox2.Text+richTextBox1.Text;
                            w.WriteLine(richTextBox2.Text);
                            w.Flush();
                        }
                    }
                    catch
                    {
                        MessageBox.Show("连接断开！！");
                    }
                }
                else
                {
                    MessageBox.Show("对不起，还没有与对方连接，不能通信！");
                }
                richTextBox2.Text = "";
                richTextBox2.Focus();
            }
        }
    }
}

客户端软件：

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;
// 添加新的命名空间。
using System.IO;
using System.Text;
using System.Net;
using System.Net.Sockets;
using System.Threading;

namespace TalkClient
{
    /// <summary>
    /// 客户端软件。
    /// </summary>
    public class Form1 : System.Windows.Forms.Form
    {
        private System.Windows.Forms.GroupBox groupBox1;
        private System.Windows.Forms.Panel panel1;
        private System.Windows.Forms.TextBox textBox1;
        private System.Windows.Forms.TextBox textBox2;
        private System.Windows.Forms.Label label1;
        private System.Windows.Forms.Label label2;
        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.RichTextBox richTextBox1;
        private System.Windows.Forms.RichTextBox richTextBox2;
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        private System.ComponentModel.Container components = null;

        public Form1()
        {
            // Windows 窗体设计器支持所必需的
            InitializeComponent();
            // TODO: 在 InitializeComponent 调用后添加任何构造函数代码
        }
        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        protected override void Dispose( bool disposing )
        {
            if( disposing )
            {
                if (components != null)
                {
                    components.Dispose();
                }
            }
            base.Dispose( disposing );
        }

        Windows Form Designer generated code

        /// <summary>
        /// 应用程序的主入口点。
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.Run(new Form1());
        }
        // 定义私有变量。
        public bool m_bConnected = false;
        public Thread t = null;
        public IPEndPoint p = null;
        public Socket s = null;
        public NetworkStream ns = null;
        public TextReader r = null;
        public TextWriter w = null;
        public void ThreadListen()
        {
            string tmp;
            while(m_bConnected)
            {
                try
                {
                    tmp = r.ReadLine();
                    if(tmp.Length != 0)
                    {
                        lock(this)
                        {
                            richTextBox1.Text = "他说："+tmp+"\n"+richTextBox1.Text;
                        }
                    }
                }
                catch
                {
                    MessageBox.Show("连接断开！！");
                }
            }
            s.Shutdown(SocketShutdown.Both);
            s.Close();
        }
        private void button1_Click(object sender, System.EventArgs e)
        {
            p = new IPEndPoint(IPAddress.Parse(textBox1.Text), int.Parse(textBox2.Text));
            s = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            s.Connect(p);
            if(s.Connected)
            {
                ns = new NetworkStream(s);
                r = new StreamReader(ns);
                w = new StreamWriter(ns);
                t = new Thread(new ThreadStart(this.ThreadListen));
                t.Start();
                m_bConnected = true;
                button1.Enabled = false;
            }
        }
        private void richTextBox2_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            if(e.KeyChar == (char)13)
            {
                if(m_bConnected)
                {
                    lock(this)
                    {
                        try
                        {
                            richTextBox1.Text = "我说："+richTextBox2.Text+richTextBox1.Text;
                            w.WriteLine(richTextBox2.Text);
                            w.Flush();
                        }
                        catch
                        {
                            MessageBox.Show("连接断开！！");
                        }
                    }
                }
                else
                {
                    MessageBox.Show("对不起，还没有与对方连接，不能通信！");
                }
                richTextBox2.Text = "";
                richTextBox2.Focus();
            }
        }
    }
}