客户端代码:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Net.Sockets;
using System.Threading;
using System.Net;

namespace p2p
{
    public partial class Form1 : Form
    {
        private Thread th;
        private UdpClient tcpl;
        public bool listenerRun = true;
        Socket s;
        //listenerRun为true，表示可以接受连接请求，false则为结束程序

public Form1()
        {
            InitializeComponent();

Control.CheckForIllegalCrossThreadCalls = false;
        }
        public void Stop()
        {
            tcpl.Close();
            th.Abort();//终止线程
            MessageBox.Show("结束监听....");
        }

private void Form1_Load(object sender, EventArgs e)
        {

}

private void button1_Click(object sender, EventArgs e)
        {
           th = new Thread(new ThreadStart(Listen));//新建一个用于监听的线程
           th.Start();//打开新线程
        }

private void Listen()
        {
            try
            {
                tcpl = new UdpClient(5656);//在5656端口新建一个TcpListener对象
                IPEndPoint groupEP = new IPEndPoint(IPAddress.Any, 5656);
                MessageBox.Show("正在监听中....");
                 while (listenerRun)//开始监听
                 {
                     Byte[] stream = new Byte[80];
                     stream = tcpl.Receive(ref groupEP);
                   
                     MessageBox.Show(groupEP.ToString() + System.Text.Encoding.UTF8.GetString(stream));               
                }
            }
            catch (Exception ex)
            {
             MessageBox.Show(ex.Message);
            }
            
        }

private void button3_Click(object sender, EventArgs e)
        {
            th = new Thread(new ThreadStart(Send));//新建一个用于监听的线程
            th.Start();//打开新线程
        }

public void Send()
        {
            try
            {
                 s = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
                 IPAddress broadcast = IPAddress.Parse(this.textBox3.Text.Trim());
                 byte[] sendbuf = Encoding.UTF8.GetBytes(this.textBox2.Text.Trim());
                IPEndPoint ep = new IPEndPoint(broadcast, 5270);
                s.Connect(ep);
                s.SendTo(sendbuf, ep);
                while (true)
                {
                    byte[] data = new byte[2048];
                    int rect = s.Receive(data);
                    byte[] chat = new byte[rect];
                    Buffer.BlockCopy(data, 0, chat, 0, rect);
                    textBox1.AppendText(System.Text.Encoding.Default.GetString(chat));
                } 
               }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);

}
            finally
            {
              
            }
            
        }

private void button2_Click(object sender, EventArgs e)
        {
            Stop();
        }

}
}

服务端代码:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Net.Sockets;
using System.Threading;
using System.Net;
using System.Collections;

namespace p2pserver
{
    public partial class Form1 : Form
    {
        private Thread th;
        private UdpClient tcpl;
        public bool listenerRun = true;
        private ArrayList pclient;//
        IPEndPoint groupEP;
        public Form1()
        {
            InitializeComponent();
            this.button2.Enabled = false;
            Control.CheckForIllegalCrossThreadCalls = false;
        }
        private void Listen()
        {
            try
            {
                tcpl = new UdpClient(5270);//在5656端口新建一个TcpListener对象
                 groupEP = new IPEndPoint(IPAddress.Any, 5270);
                 pclient = new ArrayList();
                while (listenerRun)//开始监听
                {
                    pclient.Add(groupEP);    
                    Byte[] stream = new Byte[80];
                    stream = tcpl.Receive(ref groupEP);
                    this.textBox1.AppendText(groupEP.ToString() + System.Text.Encoding.UTF8.GetString(stream));
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
            finally
            {

}
        }
        public void Stop()
        {
            try
            {
              tcpl.Close();
                th.Abort();//终止线程
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
      
        private void button1_Click(object sender, EventArgs e)
        {
            this.button1.Enabled = false;
            this.button2.Enabled = true;
            th = new Thread(new ThreadStart(Listen));//新建一个用于监听的线程
            th.Start();//打开新线程
        }

private void button2_Click(object sender, EventArgs e)
        {
            this.button2.Enabled = false;
            this.button1.Enabled = true;
            Stop();
        }

private void button3_Click(object sender, EventArgs e)
        {
            try
            {
                int index = int.Parse(this.textBox3.Text.Trim());
                byte[] bytesMsg = Encoding.Default.GetBytes(this.textBox2.Text.Trim());
                IPEndPoint sc = (IPEndPoint)pclient[index];
                tcpl.Send(bytesMsg, bytesMsg.Length, sc);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }
}