private void login_Click(object sender, RoutedEventArgs e)
        {
            if( session == null )
            {
                session = new Session();
                session.Address = ipaddress.Text;
                session.Port = Int32.Parse(port.Text);
                session.MessageHandler = OnMessage;
                session.OnClose = OnClose;
                session.OnConnection = OnConnection;
                session.Dispatcher = this.Dispatcher;

                session.StartConnection();
            }
            else
            {
                Login();
            }
        }
 void OnClose(IAsyncResult ar)
 {
     session = null;
 }