如何处理异步套接字中的超时？

Question

我有一个代码，使用异步套接字发送消息给客户端，并期待从它的响应。如果客户没有在指定的内部回复，它会认为超时。 Internet上的一些文章建议使用WaitOne，但这会阻塞该线程并推迟使用I/O完成的目的。

处理异步套接字超时的最佳方式是什么？

Sub OnSend(ByVal ar As IAsyncResult) 
     Dim socket As Socket = CType(ar.AsyncState ,Socket) 
     socket.EndSend(ar) 

     socket.BeginReceive(Me.ReceiveBuffer, 0, Me.ReceiveBuffer.Length, SocketFlags.None, New AsyncCallback(AddressOf OnReceive), socket) 

End Sub 

Answer 1

您不能超时或取消异步Socket操作。

所有你能做的就是开始自己Timer其封闭Socket -the回调将被立即调用，EndX功能会回来与ObjectDisposedException如果你调用它。以下是一个示例：

using System; 
using System.Threading; 
using System.Net.Sockets; 

class AsyncClass 
{ 
    Socket sock; 
    Timer timer; 
    byte[] buffer; 
    int timeoutflag; 

    public AsyncClass() 
    { 
      sock = new Socket(AddressFamily.InterNetwork, 
       SocketType.Stream, 
       ProtocolType.Tcp); 

      buffer = new byte[256]; 
    } 

    public void StartReceive() 
    { 
      IAsyncResult res = sock.BeginReceive(buffer, 0, buffer.Length, 
       SocketFlags.None, OnReceive, null); 

      if(!res.IsCompleted) 
      { 
       timer = new Timer(OnTimer, null, 1000, Timeout.Infinite); 
      } 
    } 

    void OnReceive(IAsyncResult res) 
    { 
      if(Interlocked.CompareExchange(ref timeoutflag, 1, 0) != 0) 
      { 
       // the flag was set elsewhere, so return immediately. 
       return; 
      } 

      // we set the flag to 1, indicating it was completed. 

      if(timer != null) 
      { 
       // stop the timer from firing. 
       timer.Dispose(); 
      } 

      // process the read. 

      int len = sock.EndReceive(res); 
    } 

    void OnTimer(object obj) 
    { 
      if(Interlocked.CompareExchange(ref timeoutflag, 2, 0) != 0) 
      { 
       // the flag was set elsewhere, so return immediately. 
       return; 
      } 

      // we set the flag to 2, indicating a timeout was hit. 

      timer.Dispose(); 
      sock.Close(); // closing the Socket cancels the async operation. 
    } 
}