How can I enforce users' input object type?

public Protocol(object port){
    Type x = port.GetType();

    if (x == typeof(SerialPort)){

    }
    else if (x == typeof(TcpClient)){

    }
    else{
       //if this is really what you need. Not good though.
       throw new Exception();
    }
}

or you could use these to avoid errors:

public Protocol (System.Net.Sockets.TcpClient tcpClient){

}

public Protocol(System.IO.Ports.SerialPort serialPort){

}