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

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)?

Solution 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

Solution 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;

*/
    }

Solution 3:

The Socket.Connected property will tell you whether a socket thinks it's connected. It actually reflects the status of the last send/receive operation performed on the socket.

If the socket has been closed by your own actions (disposing the socket, calling methods to disconnect), Socket.Connected will return false. If the socket has been disconnected by other means, the property will return true until you next attempt to send or recieve information, at which point either a SocketException or ObjectDisposedException will be thrown.

You can check the property after the exception has occurred, but it's not reliable before.

Solution 4:

The accepted answer doesn't seem to work if you unplug the network cable. Or the server crashes. Or your router crashes. Or if you forget to pay your internet bill. Set the TCP keep-alive options for better reliability.

public static class SocketExtensions
{
    public static void SetSocketKeepAliveValues(this Socket instance, int KeepAliveTime, int KeepAliveInterval)
    {
        //KeepAliveTime: default value is 2hr
        //KeepAliveInterval: default value is 1s and Detect 5 times

        //the native structure
        //struct tcp_keepalive {
        //ULONG onoff;
        //ULONG keepalivetime;
        //ULONG keepaliveinterval;
        //};

        int size = Marshal.SizeOf(new uint());
        byte[] inOptionValues = new byte[size * 3]; // 4 * 3 = 12
        bool OnOff = true;

        BitConverter.GetBytes((uint)(OnOff ? 1 : 0)).CopyTo(inOptionValues, 0);
        BitConverter.GetBytes((uint)KeepAliveTime).CopyTo(inOptionValues, size);
        BitConverter.GetBytes((uint)KeepAliveInterval).CopyTo(inOptionValues, size * 2);

        instance.IOControl(IOControlCode.KeepAliveValues, inOptionValues, null);
    }
}



// ...
Socket sock;
sock.SetSocketKeepAliveValues(2000, 1000);

The time value sets the timeout since data was last sent. Then it attempts to send and receive a keep-alive packet. If it fails it retries 10 times (number hardcoded since Vista AFAIK) in the interval specified before deciding the connection is dead.

So the above values would result in 2+10*1 = 12 second detection. After that any read / wrtie / poll operations should fail on the socket.