TCP通信需要通信双方都在线，所以需要先启动服务端进行监听，客户端才能获得连接，服务端代码：

static void Main(string[] args)        {            TcpClient client = null;            NetworkStream stream = null;            byte[] buffer = null;            string receiveString = null;            IPAddress localIP = IPAddress.Parse("127.0.0.1");            int localPort = 11000;            TcpListener listener = new TcpListener(localIP, localPort);//用本地IP和端口实例化Listener            listener.Start();//开始监听            while (true)            {                client = listener.AcceptTcpClient();//接受一个Client                buffer = new byte[client.ReceiveBufferSize];                stream = client.GetStream();//获取网络流                stream.Read(buffer, 0, buffer.Length);//读取网络流中的数据                stream.Close();//关闭流                client.Close();//关闭Client                receiveString = Encoding.Default.GetString(buffer).Trim('\0');//转换成字符串                Console.WriteLine(receiveString);            }        }

只有服务端开启监听后，客户端才能正确连接，所以服务端要一直开启监听，客户端每次发送数据，都要首先与服务端建立连接，连接建立完成后才进行数据发送。客户端代码：

static void Main(string[] args)        {            string sendString = null;//要发送的字符串            byte[] sendData = null;//要发送的字节数组            TcpClient client = null;//TcpClient实例            NetworkStream stream = null;//网络流            IPAddress remoteIP = IPAddress.Parse("127.0.0.1");//远程主机IP            int remotePort = 11000;//远程主机端口            while (true)//死循环            {                sendString = Console.ReadLine();//获取要发送的字符串                sendData = Encoding.Default.GetBytes(sendString);//获取要发送的字节数组                client = new TcpClient();//实例化TcpClient                try                {                    client.Connect(remoteIP, remotePort);//连接远程主机                }                catch (System.Exception ex)                {                    Console.WriteLine("连接超时，服务器没有响应！");//连接失败                    Console.ReadKey();                    return;                }                stream = client.GetStream();//获取网络流                stream.Write(sendData, 0, sendData.Length);//将数据写入网络流                stream.Close();//关闭网络流                client.Close();//关闭客户端            }        }