Rectangle.Offset method have no effect?

I have this piece of code in my super simple drawing program over here:

if (isMovingRectangle())
{
    selectedRectangle.BoundingBox.Offset(
    currentMousePos.X - mousePosWhenDown.X,
    currentMousePos.Y - mousePosWhenDown.Y);

    Invalidate();
}

I execute these statements every time when the MouseMove event occurs, but why doesn't the Offset method effect the location of the selectedRectangle???? Thank you so much for your answers.