TCP Socket一端关闭之后不报异常，而是接受0字节。

Question

我用.net 3.5, VS 2008 开发， TCP Socket通讯，递归调用接受信息。在实际项目中发现当 Socket一端关闭（调用Socket.Close())后，另外一端不是报出异常（我期望是如此），而是不断递归调用接受0字节。我整理了下代码，下面Demo能重现此问题。期望有人能给出解释。
服务器端代码    class TestServer
    {
        static void Main()
        {
            try
            {
                TCPServer tcpServer = (TCPServer)TCPServer.GetInstance();
                tcpServer.ServerIP = "127.0.0.1";
                tcpServer.ServerPort = 3721;
                tcpServer.CreatListener();
                Console.WriteLine(String.Format("Server is ready with {0}:{1}", tcpServer.ServerIP, tcpServer.ServerPort));
            }           
            catch (Exception e)
            {
                Console.WriteLine("Exception: {0}", e);
            }
            Console.WriteLine("Press Enter to Exit"); Console.ReadLine();
        }    
    }     public abstract class PacketTransferBase
    {
        public PacketTransferBase()
        {            
        }
        private int count = 0;
        /// <summary>
        /// 接收信息
        /// </summary>
        /// <param name="ar"></param>
        protected void ReceiveMessage(IAsyncResult ar)
        {
            try
            {
                StateObject so = (StateObject)ar.AsyncState;
                Socket socket = so.workSocket;                                //方法参考：http://msdn.microsoft.com/zh-cn/library/system.net.sockets.socket.endreceive.aspx
                var length = socket.EndReceive(ar);
                byte[] actualData = new byte[length];
                Array.Copy(so.buffer, 0, actualData, 0, length);
               
                Console.WriteLine(string.Format("TCPServer.ReceiveMessage: buffer.{0} at Socket.{1}.{2} with buffer[0].{3},buffer[1].{4},buffer[2].{5}", length, socket.Handle, socket.Connected, so.buffer[0], so.buffer[1], so.buffer[2]));                bool continueReceive = true;
                Console.WriteLine(Encoding.UTF8.GetString(actualData));
                if (count == 0)
                {
                    socket.Send(Encoding.UTF8.GetBytes("this is a back message from server!"));
                    count++;
                }
                socket.Close();                if (continueReceive && socket != null && socket.Connected)
                {
                    socket.BeginReceive(so.buffer, 0, StateObject.BUFFER_SIZE, 0, new AsyncCallback(ReceiveMessage), so);
                }                
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message + ex.StackTrace);
            }
        }
    }        /// <summary>
    /// TCP服务器端
    /// </summary>
    public class TCPServer : PacketTransferBase
    {
        public string ServerIP { get; set; }
        public int ServerPort { get; set; }     
        private static TCPServer _TCPServer = new TCPServer();
        Socket socketServer = null;
       
        private TCPServer()
        {            
        }        public static PacketTransferBase GetInstance()
        {
            return _TCPServer;
        }        /// <summary>
        /// 服务器端建立侦听
        /// </summary>
        public void CreatListener()
        {
            //创建一个新的Socket,这里我们使用最常用的基于TCP的Stream Socket（流式套接字）
            socketServer = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);            //将该socket绑定到主机上面的某个端口
            //方法参考：http://msdn.microsoft.com/zh-cn/library/system.net.sockets.socket.bind.aspx           
            socketServer.Bind(new IPEndPoint(IPAddress.Parse(ServerIP), ServerPort));            //启动监听，并且设置一个最大的队列长度
            //方法参考：http://msdn.microsoft.com/zh-cn/library/system.net.sockets.socket.listen(v=VS.100).aspx
            socketServer.Listen(100);            //开始接受客户端连接请求
            //方法参考：http://msdn.microsoft.com/zh-cn/library/system.net.sockets.socket.beginaccept.aspx
            socketServer.BeginAccept(new AsyncCallback(ClientAccepted), socketServer);
        }        /// <summary>
        /// 接收客户端
        /// </summary>
        /// <param name="ar"></param>
        public void ClientAccepted(IAsyncResult ar)
        {
            try
            {
                var socketServer = ar.AsyncState as Socket;                //这就是客户端的Socket实例，我们后续可以将其保存起来
                var socketClient = socketServer.EndAccept(ar);                                //接收客户端的消息(这个和在客户端实现的方式是一样的）                
                StateObject so = new StateObject();
                so.workSocket = socketClient;
                socketClient.BeginReceive(so.buffer, 0, StateObject.BUFFER_SIZE, 0, new AsyncCallback(ReceiveMessage), so);                //准备接受下一个客户端请求
                socketServer.BeginAccept(new AsyncCallback(ClientAccepted), socketServer);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message + ex.StackTrace);
            }
        }
    }    public class StateObject
    {
        public Socket workSocket = null;
        public EndPoint tempRemoteEP = null;
        public const int BUFFER_SIZE = 1024;
        public byte[] buffer = new byte[BUFFER_SIZE];
    }
客户端代码    class TestClient
    {
        static void Main()
        {
            try
            {
                TCPClient tcpClient = (TCPClient)TCPClient.GetInstance();
                tcpClient.ServerIP = "127.0.0.1";
                tcpClient.ServerPort = 3721;                               Socket socket = tcpClient.ConnectToServer();
                socket.Send(Encoding.UTF8.GetBytes("this is a test message from client!"));                
            }            
            catch (Exception e)
            {
                Console.WriteLine("Exception: {0}", e);
            }
            Console.WriteLine("Press Enter to Exit"); Console.ReadLine();
        }         
    }    public abstract class PacketTransferBase
    {
        public PacketTransferBase()
        {
        }        private int count = 0;
        /// <summary>
        /// 接收信息
        /// </summary>
        /// <param name="ar"></param>
        protected void ReceiveMessage(IAsyncResult ar)
        {
            try
            {
                StateObject so = (StateObject)ar.AsyncState;
                Socket socket = so.workSocket;                //方法参考：http://msdn.microsoft.com/zh-cn/library/system.net.sockets.socket.endreceive.aspx
                var length = socket.EndReceive(ar);
                byte[] actualData = new byte[length];
                Array.Copy(so.buffer, 0, actualData, 0, length);                Console.WriteLine(string.Format("TCPServer.ReceiveMessage: buffer.{0} at Socket.{1}.{2} with buffer[0].{3},buffer[1].{4},buffer[2].{5}", length, socket.Handle, socket.Connected, so.buffer[0], so.buffer[1], so.buffer[2]));                bool continueReceive = true;
                Console.WriteLine(Encoding.UTF8.GetString(actualData));
                //把以下代码去掉注释，把服务器端socket.Close()注释，则服务器端产生一样效果。
                //if (count == 0)
                //{
                //    socket.Send(Encoding.UTF8.GetBytes("this is a second message from server!"));
                //    count++;
                //}
                //socket.Close();                if (continueReceive && socket != null && socket.Connected)
                {
                    socket.BeginReceive(so.buffer, 0, StateObject.BUFFER_SIZE, 0, new AsyncCallback(ReceiveMessage), so);
                }
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message + ex.StackTrace);
            }
        }
    }        public class StateObject
    {
        public Socket workSocket = null;
        public EndPoint tempRemoteEP = null;
        public const int BUFFER_SIZE = 1024;
        public byte[] buffer = new byte[BUFFER_SIZE];
    }
    /// <summary>
    /// TCP 客户端
    /// </summary>
    public class TCPClient : PacketTransferBase
    {
        public string ServerIP { get; set; }
        public int ServerPort { get; set; }               private static TCPClient _TCPClient = new TCPClient();               private TCPClient()
        {            
        }        public static PacketTransferBase GetInstance()
        {
            return _TCPClient;
        }                public Socket ConnectToServer()
        {
            //创建一个Socket
            Socket socketClient = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);            //连接到指定服务器的指定端口
            //方法参考：http://msdn.microsoft.com/zh-cn/library/system.net.sockets.socket.connect.aspx           
            socketClient.Connect(ServerIP, ServerPort);            StateObject so = new StateObject();
            so.workSocket = socketClient;
            socketClient.BeginReceive(so.buffer, 0, StateObject.BUFFER_SIZE, 0, new AsyncCallback(ReceiveMessage), so);
            return socketClient;            
        }
    }
服务端端和客户端关闭Socket部分互换后（注释服务器端，开启客户端关闭），服务器端也能长生此同样现象。socket

Answer 1

这可能算是一个BUG吧。理论上是应该阻塞的。
还有更多的问题。
codeproject有一个开源东东很多也碰到过原子锁会出现负的情况。使用心跳包吧。

Answer 2

我是这么做的，发现这个情形就是在一种情况有一边忘记关了，就出现上面的情况。。很奇怪。没查到有用的文档。这个也不像是 Keep-Alive数据报啊。那个应该没这么频繁吧。。

Answer 3

是的，要进行处理。不过我查论坛，或者msdn都没发现此情形，难道大家都没碰到过这种情况