服务器端：
using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using
using
using System.Threading;

namespace 异步通信服务器
{
    public partial class Form1 : Form
    {
        private Socket serversocket,clientsocket;
        private const int dataSize=1024;
        private byte[] data = new byte[dataSize];
            
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.button1.Enabled = false;
            IPAddress ip = IPAddress.Parse("127.0.0.1");
            IPEndPoint server = new IPEndPoint(ip, 8000);
            serversocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            serversocket.Bind(server);
            serversocket.Listen(10);
            serversocket.BeginAccept(new AsyncCallback(AcceptConnection), serversocket);
        }
        private void AcceptConnection(IAsyncResult ar)
        {
            Socket oldServer = (Socket)ar.AsyncState;
            //异步接受传入的连接，并创建新的Socket来处理远程主机通信
            clientsocket = oldServer.EndAccept(ar);
            MessageBox.Show("与客户建立连接！");
            clientsocket.BeginReceive(data, 0, dataSize, SocketFlags.None, new AsyncCallback(ReceiveData), clientsocket);
        }
        private void ReceiveData(IAsyncResult ar)
        {
            Socket client = (Socket)ar.AsyncState;//client改成socketclient？？？
            try//就是这里，接收失败，显示“等待新客户！”
            {
                int receiveDataLength = client.EndReceive(ar);
                string str = System.Text.Encoding.Unicode.GetString(data, 0, receiveDataLength);
                this.richTextBox1.Text = str;
                clientsocket.BeginReceive(data, 0, dataSize, SocketFlags.None, new AsyncCallback(ReceiveData), clientsocket);//因为接收后还要继续准备接收，这样可以麽？？？
            }
            catch
            {
                client.Close();
                MessageBox.Show("等待新客户！");
                serversocket.BeginAccept(new AsyncCallback(AcceptConnection), serversocket);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            try
            {
                serversocket.Close();
                MessageBox.Show("停止监听！");
            }
            catch (Exception err)
            {
                MessageBox.Show(err.Message,"错误");
            }

        }
    }
}



客户端：
using System;
using System.Collections.Generic;
using
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using
using
using System.Threading;


namespace 异步通信客户端
{
    public partial class Form1 : Form
    {
        private Socket clientsocket;
        private const int datasize = 1024;
        private byte[] data = new byte[datasize];
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            IPAddress ip = IPAddress.Parse("127.0.0.1");
            IPEndPoint server = new IPEndPoint(ip, 8000);
            Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            socket.BeginConnect(server, new AsyncCallback(ConnectServer), socket);
        }
        private void ConnectServer(IAsyncResult ar)
        {
            clientsocket = (Socket)ar.AsyncState;
            try
            {
                clientsocket.EndConnect(ar);
                MessageBox.Show("与服务器连接成功！");
                clientsocket.BeginReceive(data, 0, datasize, SocketFlags.None, new AsyncCallback(ReceiveData), clientsocket);
            }
            catch
            {
                MessageBox.Show("与服务器连接失败！");
            }

        }
        private void ReceiveData(IAsyncResult ar)
        {
            try
            {
                Socket server = (Socket)ar.AsyncState;
                int receiveDataLength = server.EndReceive(ar);
                string str = System.Text.Encoding.Unicode.GetString(data, 0, receiveDataLength);
                this.richTextBox1.Text = str;
            }
            catch
            { }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            try
            {
                int bytes = 0;
                Byte[] SendBytes = new Byte[256];
                string SendString = "123#";
                SendBytes = Encoding.ASCII.GetBytes(SendString.ToCharArray());
                bytes = clientsocket.Send(SendBytes, SendBytes.Length, 0);

             }
            catch
            {
                MessageBox.Show("发送失败！");
            }
        }
        
        private void button3_Click(object sender, EventArgs e)
        {
            try
            {
                clientsocket.Close();
                MessageBox.Show("与服务器断开连接！");
            }
            catch
            {
                MessageBox.Show("连接尚未开始");
            }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            try
            {
                int bytes = 0;
                Byte[] SendBytes = new Byte[256];
                string SendString = "456#";
                SendBytes = Encoding.ASCII.GetBytes(SendString.ToCharArray());
                bytes = clientsocket.Send(SendBytes, SendBytes.Length, 0);

             }
            catch
            {
                MessageBox.Show("发送失败！");
            }
        }
        
    }
}