How to tell when a Socket has been disconnected

Question

On the client side I need to know when/if my socket connection has been broken. However the Socket.Connected property always returns true, even after the server side has been d

Answer 1

After doing some testing, it appears that the documentation for Socket.Connected is wrong, or at least misleading. clientSocket.Connected will only become false after clientSocket.close() is called. I think this is a throwback to the original C Berkeley sockets API and its terminology. A socket is bound when it has a local address associated with it, and a socket is connected when it has a remote address associated with it. Even though the remote side has closed the connection, the local socket still has the association and so it is still "connected".

However, here is a method that does work:

!(socket.Poll(0, SelectMode.SelectRead) && socket.Available == 0)

It relies on that fact that a closed connection will be marked as readable even though no data is available.

If you want to detect conditions such as broken network cables or computers abruptly being turned off, the situation is a bit more complex. Under those conditions, your computer never receives a packet indicating that the socket has closed. It needs to detect that the remote side has vanished by sending packets and noticing that no response comes back. You can do this at the application level as part of your protocol, or you can use the TCP KeepAlive option. Using TCP Keep Alive from .NET isn't particularly easy; you're probably better off building a keep-alive mechanism into your protocol (alternately, you could ask a separate question for "How do I enable TCP Keep Alive in .NET and set the keep alive interval?").