socket 服务端为什么会随客户端关闭需关闭?

服务器端部分代码如下,为什么客户端连接上后一关闭服务器端也会关闭?
private void Form1_Load(object sender, EventArgs e)
        {
            this.transPort = "88";
            System.Windows.Forms.Control.CheckForIllegalCrossThreadCalls = false;//新加防止出现“线程间操作无效: 从不是创建控件“listBox1”的线程访问它”类错误。
        }        private void btnCreate_Click(object sender, EventArgs e)
        {
            Thread acceptThread = new Thread(new ThreadStart(BeginListen));
            acceptThread.Start();
            this.BeginListen();
        }
        
        //开始监听
        void BeginListen()
        {
            this.Text = "在端口: " + this.transPort + " 监听...";
            try
            {
                IPHostEntry hostEntry = null;
                hostEntry = Dns.GetHostEntry("10.30.11.201");
                this.mainSocketTest = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
                IPEndPoint ipe = new IPEndPoint(hostEntry.AddressList[0], int.Parse(this.transPort));
                //IPEndPoint ipe = new IPEndPoint(Dns.GetHostEntry(Dns.GetHostName()).AddressList[0], int.Parse(this.transPort));                this.mainSocketTest.SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, true);//端口复用++新加的                this.mainSocketTest.Bind(ipe);
                this.mainSocketTest.Listen(10);                this.netDelegate = new NetDelegate(this.CatchConnection);
                this.asynCallDelegate = new AsyncCallback(this.EndCall);
                this.netDelegate.BeginInvoke(this.asynCallDelegate, (object)"ddd");
            }
            catch (Exception e)
            {
                this.CloseAll();////////////////////////////////////////////++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
                MessageBox.Show(e.Message+"1");
                return;
            }
        }