How to check if a socket is connected/disconnected in C#?

Question

How can you check if a network socket (System.Net.Sockets.Socket) is still connected if the other host doesn't send you a packet when it disconnects (e.g. because it disconnected ungracefully)?

Answer 1

As Paul Turner answered Socket.Connected cannot be used in this situation. You need to poll connection every time to see if connection is still active. This is code I used:

bool SocketConnected(Socket s) {     bool part1 = s.Poll(1000, SelectMode.SelectRead);     bool part2 = (s.Available == 0);     if (part1 && part2)         return false;     else         return true; } 

It works like this:

• s.Poll returns true if
  • connection is closed, reset, terminated or pending (meaning no active connection)
  • connection is active and there is data available for reading
• s.Available returns number of bytes available for reading
• if both are true:
  • there is no data available to read so connection is not active

Answer 2

As zendar wrote, it is nice to use the Socket.Poll and Socket.Available, but you need to take into consideration that the socket might not have been initialized in the first place. This is the last (I believe) piece of information and it is supplied by the Socket.Connected property. The revised version of the method would looks something like this:

 static bool IsSocketConnected(Socket s)     {         return !((s.Poll(1000, SelectMode.SelectRead) && (s.Available == 0)) || !s.Connected);  /* The long, but simpler-to-understand version:          bool part1 = s.Poll(1000, SelectMode.SelectRead);         bool part2 = (s.Available == 0);         if ((part1 && part2 ) || !s.Connected)             return false;         else             return true;  */     }