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 discon

Answer 1

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.