Determining if .NET socket is currently connected bug?

Question

Hi,

I'm trying to find out whether a Socket is currently connected - according to the MSDN docs for Socket.Connected - I should do the following:

// This is how you can determine whether a socket is still connected.
bool blockingState = client.Blocking;
try
{
    byte [] tmp = new byte[1];

    client.Blocking = false;
    client.Send(tmp, 0, 0);
    Console.WriteLine("Connected!");
}
catch (SocketException e) 
{
    // 10035 == WSAEWOULDBLOCK
    if (e.NativeErrorCode.Equals(10035))
        Console.WriteLine("Still Connected, but the Send would block");
    else
    {
        Console.WriteLine("Disconnected: error code {0}!", e.NativeErrorCode);
    }
}
finally
{
    client.Blocking = blockingState;
}

 Console.WriteLine("Connected: {0}", client.Connected);

I've tested this works by connecting the socket to a remote server running on Windows and killing the remote server and it works fine.

However, if I do the same with a remote Server running on Unix (in this case MAC OS X) then the code does not work - the client.Send(tmp, 0, 0) call completes without throwing an exception and prints "Connected: true".

I'm guessing this has something to do with the fact that the other side of the connection has closed so the send may still work but a receive would fail - can I do a zero-byte Receive or something to see if the socket is closed?

Answer 1

Yes, you can.

Calling Send only really checks if the local socket is open. Calling Receive will check the other end too.

Answer 2

Maybe you can sniff with Wireshark the traffic and see if there are any difference if the dying server side is a windows or unix system.

Maybe the dying windows system is able to send a tcp close while the unix system can't. That would explain the difference (but maybe not help your actual problem).

btw, if you would use a udp connection you could never determine if the other site is living, due to the fact this would be a send & forget communication.