Socket端口监听无法关闭，那位高人给指点1and2

public Socket mySocket;
        public TcpListener listener;
        public NetworkStream netStream;
        public IPEndPoint IPE;
        public Thread thread;
        public byte[] ReveiceData = new byte[200];
        public bool Control = true;        public void StartListening()
        {
            try
            {
                int Port = 4000;
                IPAddress IP = IPAddress.Parse("192.168.1.27");
                IPE = new IPEndPoint(IP, Port);
                listener = new TcpListener(IPE);
                listener.Start();
                this.Text = "监听状态:   开启.";
                Thread ListenThread = new Thread(new ThreadStart(clientConnect));
                ListenThread.Start();
            }
            catch (Exception ListenError)
            { MessageBox.Show(ListenError.Message); }
        }        public void clientConnect()
        {
            while (Control)
            {
                mySocket = listener.AcceptSocket();
                if (mySocket.Connected)
                {
                    this.Text = "         连接状态:   客户端建立连接.";
                    Thread thread = new Thread(new ThreadStart(Round));
                    thread.Start();
                }
                else this.Text = "         连接状态:   客户端断开连接.";
            }
        }        public void Round()
        {
            while (true)
            {
                netStream = new NetworkStream(mySocket);
                netStream.Read(ReveiceData, 0, ReveiceData.Length);
                this.BeginInvoke(new ShowDelegate(DataShow));
            }
        }        public delegate void ShowDelegate();
        public void DataShow()
        {
            this.Text = "OK";
        }        public void StopListen()
        {
            try
            {
                Control = false;
                mySocket.Close();
                listener.Stop();
                this.Text = "监听状态:   关闭.";
            }
            catch (Exception StopError)
            { MessageBox.Show(StopError.Message); }
        }        private void button1_Click(object sender, EventArgs e)
        {
            StopListen();
        }        private void button2_Click(object sender, EventArgs e)
        {
            StartListening();
        }错误提示：未将对象引用设置到对象的实例，不明白？？？