boolean mockup

bool topleft = false;
bool topright = false;
bool bottomleft = false;
bool bottomright = false;

private void handleTopRight_MouseDown(object sender, MouseEventArgs e)
{
    topRight = true;
}

private void handleTopRight_MouseUp(object sender, MouseEventArgs e)
{
    topRight = false;
}

private void handleTopRight_MouseMove(object sender, MouseEventArgs e)
{
    if(topRight)
    {
        // Do move code...
    }
}