In my WinSharer Application,

private void OnAttendeeConnected(object pObjAttendee)
{
         IRDPSRAPIAttendee pAttendee = pObjAttendee as IRDPSRAPIAttendee;
         pAttendee.ControlLevel = CTRL_LEVEL.CTRL_LEVEL_VIEW;
         //MessageBox.Show(("Attendee Connected: " + pAttendee.RemoteName + Environment.NewLine));
}

void OnAttendeeDisconnected(object pDisconnectInfo)
{
            IRDPSRAPIAttendeeDisconnectInfo pDiscInfo = pDisconnectInfo as IRDPSRAPIAttendeeDisconnectInfo;  
}

void OnControlLevelChangeRequest(object pObjAttendee, CTRL_LEVEL RequestedLevel)
{
            IRDPSRAPIAttendee pAttendee = pObjAttendee as IRDPSRAPIAttendee;
            pAttendee.ControlLevel = RequestedLevel;
}

And in the WinViewer,

public void ControlDesktop(object sender, EventArgs e)
{
            desktopViewer.RequestControl(RDPCOMAPILib.CTRL_LEVEL.CTRL_LEVEL_INTERACTIVE);
}

The above is callback when the button ControlDesktop is clicked