A question about ControlPaint.DrawReversibleFrame

private void Form1_MouseMove(object sender,
    System.Windows.Forms.MouseEventArgs e)
{

    // If the mouse is being dragged,
    // undraw and redraw the rectangle as the mouse moves.
    if (isDrag)

        // Hide the previous rectangle by calling the
        // DrawReversibleFrame method with the same parameters.
    {
        ControlPaint.DrawReversibleFrame(theRectangle,
            this.BackColor, FrameStyle.Dashed);

        // Calculate the endpoint and dimensions for the new
        // rectangle, again using the PointToScreen method.
        Point endPoint = ((Control) sender).PointToScreen(new Point(e.X, e.Y));

        int width = endPoint.X-startPoint.X;
        int height = endPoint.Y-startPoint.Y;
        theRectangle = new Rectangle(startPoint.X,
            startPoint.Y, width, height);

        // Draw the new rectangle by calling DrawReversibleFrame
        // again.
        ControlPaint.DrawReversibleFrame(theRectangle,
            this.BackColor, FrameStyle.Dashed);
    }
}