同步线程和异步线程的使用,回调函数的意义

/// <summary>
         /// 应用程序的主入口点。
         /// </summary>
         [STAThread]
         static void Main() 
         {
             Application.Run(new frmServer());
         }        private void frmServer_Load(object sender, System.EventArgs e)
         {
             txtServer.Text = Dns.GetHostName();
         }        监听#region 监听
        //开始监听
         private void btnListen_Click(object sender, System.EventArgs e)
         {
             try
             {
                 IPHostEntry myHost = new IPHostEntry();
                 myHost = Dns.GetHostByName(txtServer.Text);
                 string IPstring = myHost.AddressList[0].ToString();
                 myIP = IPAddress.Parse(IPstring);
             }
             catch
             {
                 MessageBox.Show("您输入的IP地址格式不正确，请重新输入！");
             }            try
             {
                 myServer = new IPEndPoint(myIP,Int32.Parse(txtPort.Text.Trim()));
                 mySocket = new Socket(AddressFamily.InterNetwork,SocketType.Stream,ProtocolType.Tcp);
                 mySocket.Bind(myServer);
                 mySocket.Listen(50);                //IPEndPoint aaIP = (IPEndPoint)mySocket.LocalEndPoint;
                 //int aa = aaIP.Port;
                 //MessageBox.Show(aa.ToString());                txtState.AppendText("主机"+txtServer.Text+"端口"+txtPort.Text+"开始监听..\r\n");
                //线程
                 Thread thread = new Thread(new ThreadStart(target));
                 thread.Start();
             }
             catch(Exception ee)
             {
                 txtState.AppendText(ee.Message+"\r\n");
             }
         }        //线程同步方法target
         private void target()
         {
             while(true)
             {
                 //设为非终止
                 myReset.Reset();                mySocket.BeginAccept(new AsyncCallback(AcceptCallback),mySocket);
                //阻塞当前线程，直到收到请求信号
                 myReset.WaitOne();
             }
         }        //异步回调方法AcceptCallback
         private void AcceptCallback(IAsyncResult ar)
         {
             //将事件设为终止
             myReset.Set();            Socket listener = (Socket)ar.AsyncState;
             handler = listener.EndAccept(ar);            //获取状态
             StateObject state = new StateObject();
             state.workSocket = handler;            txtState.AppendText("与客户建立连接。\r\n");
            try
             {
                 byte[] byteData = System.Text.Encoding.BigEndianUnicode.GetBytes("已经准备好，请通话！"+"\r\n");
                 //开始发送
                 handler.BeginSend(byteData,0,byteData.Length,0,new AsyncCallback(SendCallback),handler);
             }
             catch(Exception ee)
             {
                 MessageBox.Show(ee.Message);
             }            //线程
             Thread thread = new Thread(new ThreadStart(begReceive));
             thread.Start();
         }        //异步回调方法 SendCallback
         private void SendCallback(IAsyncResult ar)
         {
             try
             {
                 handler = (Socket)ar.AsyncState;
                 int bytesSent = handler.EndSend(ar);
             }
             catch(Exception ee)
             {
                 MessageBox.Show(ee.Message);
             }
         }        //线程同步方法begReceive
         private void begReceive()
         {
             StateObject state = new StateObject();
             state.workSocket = handler;
             handler.BeginReceive(state.buffer,0,StateObject.BufferSize,0,new AsyncCallback(ReadCallback),state);
         }        //异步回调方法ReadCallback
         private void ReadCallback(IAsyncResult ar)
         {
             StateObject state = (StateObject) ar.AsyncState;
             Socket tt = state.workSocket;            //结束读取并获取读取字节数
             int bytesRead = handler.EndReceive(ar);
             state.sb.Append(System.Text.Encoding.BigEndianUnicode.GetString(state.buffer,0,bytesRead));
             string content = state.sb.ToString();
             state.sb.Remove(0,content.Length);
             rtbIncept.AppendText(content+"\r\n");            //重新开始读取数据
             tt.BeginReceive(state.buffer,0,StateObject.BufferSize,0,new AsyncCallback(ReadCallback),state);
         }#endregion 监听
        //发送信息
         private void btnSend_Click(object sender, System.EventArgs e)
         {
             try
             {
                 byte[] byteData = System.Text.Encoding.BigEndianUnicode.GetBytes(rtbSend.Text+"\r\n");                //开始发送数据
                 handler.BeginSend(byteData,0,byteData.Length,0,new AsyncCallback(SendCallback),handler);
             }
             catch(Exception ee)
             {
                 MessageBox.Show(ee.Message);
             }
         }        //停止监听
         private void btnStop_Click(object sender, System.EventArgs e)
         {
             try
             {
                 mySocket.Close();
                 txtState.AppendText("主机"+txtServer.Text+"端口"+txtPort.Text+"停止监听"+"\r\n");
             }
             catch
             {
                 MessageBox.Show("监听尚未开始，关闭无效！");
             }
         }    }