如何确定tcp是否已连接？

Question

我有tcpclient对象,我想确定它是否已连接.我使用tcpclient的连接属性,但它返回上一次操作的状态.所以它没用.

那我用这个代码:

bool flag;
flag = (tcp.Client.Poll(10000, SelectMode.SelectWrite));

和

 if( tcp.Client.Poll( 0, SelectMode.SelectRead ) )
 {
   byte[] buff = new byte[1];
   if( tcp.Client.Receive( buff, SocketFlags.Peek ) == 0 )
   {
     flag = false;
   }
 }

但它无法正常工作.

任何的想法？

---

这是我在服务器端的代码:

   private ArrayList _ClientList = new ArrayList();

   public ClsServer(int port)
    {
        _TCPListener = new TcpListener(IPAddress.Any, port);
        _TCPListener.Start();

        Thread ListenThread = new Thread(new ThreadStart(ListenForClients));
        ListenThread.IsBackground = true;
        ListenThread.Start();
    }

    private void ListenForClients()
    {            
        while (true)
        {
            //blocks until a client has connected to the server
            TcpClient client = this._TCPListener.AcceptTcpClient();
            client.ReceiveTimeout = 0;

            //create a thread to handle communication with connected client
            Thread clientThread = new Thread(new ParameterizedThreadStart(HandleClientComm));
            clientThread.IsBackground = true;
            clientThread.Start(client);
        }
    }

    private void HandleClientComm(object client)
    {
        try
        {
            TcpClient tcpClient = (TcpClient)client;               
            AddObject(tcpclient);

            int bytesRead;
            string message = "";
            byte[] RecievedPack = new byte[1024 * 1000];

            NetworkStream clientStream = tcpClient.GetStream();
            while (true)
            {
                bytesRead = 0;
                try
                {
                    ////blocks until a client sends a message
                    bytesRead = clientStream.Read(RecievedPack, 0, RecievedPack.Length);
                    int Len = BitConverter.ToInt32(RecievedPack, 0);
                    message = UTF8Encoding.UTF8.GetString(RecievedPack, 0, Len);
                }
                catch (Exception er)
                {
                    //When Client is disconnected
                    if (er.GetType() == typeof(IOException))
                    {
                        RemoveObject(client);
                        break;
                    }
                }                   
                //message has successfully been received                          
               // do something
            }

            RemoveObject(client);
        }
        catch(Exception e)
        {
           // RemoveObject(client);
        }
    }

    private void AddObject(object obj)
    {            
        int totalcount, index;
        totalcount = _ClientList.Count;
        index = 0;
        while (index < totalcount)
        {
            TcpClient alcobj = (TcpClient)_ClientList[index];
            try
            {
                if (IPAddress.Equals(((IPEndPoint)alcobj.Client.RemoteEndPoint).Address,
                   ((IPEndPoint)tcpClient.Client.RemoteEndPoint).Address))
                {
                    _ClientList.Remove(alcobj);
                    break;
                }
                index++;
            }
            catch (Exception er)
            {
                if (er.GetType() == typeof(ObjectDisposedException))
                    RemoveObject(alcobj);
            }
            finally
            {
                totalcount = _ClientList.Count;
            }
        }            
         _ClientList.Add(obj);             
    }

    private void RemoveObject(object obj)
    {            
        if (_ClientList.IndexOf(obj) > -1)
        {
            _ClientList.Remove(obj);
            SendClientState(IP, false);
        }         
    }

这是客户端:

    public bool IsConnected
    {           
            try
            {
                if (_TcpClient != null && _TcpClient.Client != null && _TcpClient.Client.Connected)
                {
                    // Detect if client disconnected
                    if (_TcpClient.Client.Poll(0, SelectMode.SelectRead))
                    {
                        byte[] buff = new byte[1];
                        if (_TcpClient.Client.Receive(buff, SocketFlags.Peek) == 0)
                        {
                            // Client disconnected
                            return false;
                        }
                        else
                        {
                            return true;
                        }
                    }

                    return true;
                }
                else
                {
                    return false;
                }
            }
            catch
            {
                return false;
            }
    }

   private void clsClient()
   {
          if(!IsConnected()) 
          {
                  Connecttoserver()
           }
    }

    private void ConnectToServer()
    {
        try
        {
            NetworkStream _NetworkStream = _TcpClient.GetStream();
            byte[] _RecievedPack = new byte[1024 * 1000];
            string _Message = string.Empty;
            int _BytesRead;
            int _Length;

            while (true)
            {
                _BytesRead = _NetworkStream.Read(_RecievedPack, 0, _RecievedPack.Length);
                _Length = BitConverter.ToInt32(_RecievedPack, 0);
                _Message = UTF8Encoding.UTF8.GetString(_RecievedPack, 4, _Length);

                if (_BytesRead != 0)
                {
                    if (OnReceive != null)
                        // do something

                    _NetworkStream.Flush();
                }
            }
        }
        catch (Exception exp)
        {
            // do something 
        }
    }

在客户端,IsConnected()始终返回false并尝试connecttoserver,因此服务器侦听器总是尝试在列表中添加客户端

Answer 1

使用此代码,我已测试并在实际生产软件中使用它:

public bool IsConnected
{
    get
    {
        try
        {
            if (_tcpClient != null && _tcpClient.Client != null && _tcpClient.Client.Connected)
            {
               /* pear to the documentation on Poll:
                * When passing SelectMode.SelectRead as a parameter to the Poll method it will return 
                * -either- true if Socket.Listen(Int32) has been called and a connection is pending;
                * -or- true if data is available for reading; 
                * -or- true if the connection has been closed, reset, or terminated; 
                * otherwise, returns false
                */

                // Detect if client disconnected
                if (_tcpClient.Client.Poll(0, SelectMode.SelectRead))
                {
                    byte[] buff = new byte[1];
                    if (_tcpClient.Client.Receive(buff, SocketFlags.Peek) == 0)
                    {
                        // Client disconnected
                        return false;
                    }
                    else
                    {
                        return true;
                    }
                }

                return true;
            }
            else
            {
                return false;
            }
        }
        catch
        {
            return false;
        }
    }
}

编辑:但是你不能只依赖于检查连接,如果是,则继续执行,因为它在执行此属性时返回连接状态,例如在您检查IsConnected并返回true之后,当您处于通信中间时,连接可能会丢失!我们首先使用它来降低失败的可能性,所以你必须将整个通信包装在try/catch中,并期望连接随时丢失!

Answer 2

我建议不要转发这种方法.对于我看来,最好的方法是实现某种保持活动机制.每隔X秒,发送一条小消息并等待回答.您可能在以下情况下断开连接:1.在尝试发送保持活动消息时捕获异常(如果您是客户端).2.在一段时间内,您没有收到保持活动的消息/响应.

我还建议不要指望内置的TCP keep-alive,我发现它非常不可靠.

更新:找到了一个很好的帖子:发布