Question

        using (var context = new Context(1))
        {
            using (Socket socket = context.Socket(SocketType.XREQ))
            {

                socket.SetSockOpt(SocketOpt.LINGER, 100ul);

                socket.Connect("tcp://somewhere:5566");


                string content = "test message";

                socket.Send(content, Encoding.Unicode);
                string message = socket.Recv(Encoding.Unicode);
                Console.WriteLine("Received reply: " + message);
            }
        }

I’ve tried longs, ulongs and byte arrays and I can not find a value that does not lead to an “illegal argument” exception being thrown.

Answer 1

As I can see from the clrzmq2 library sources (Socket.cs) there are 4 overloads of the SetSocketOpt() method with long, ulong, byte array or integer parameter. To specify linger period for socket shutdown you shoud use integer parameter.
Anyway, I suggest you use the Socket.Linger property to avoid any possible dismissing.