private void AddShadow(PaintEventArgs e)
{

    // Create two SizeF objects.
    SizeF shadowSize = listBox1.Size;
    SizeF addSize = new SizeF(10.5F, 20.8F);

    // Add them together and save the result in shadowSize.
    shadowSize = shadowSize + addSize;

    // Get the location of the ListBox and convert it to a PointF.
    PointF shadowLocation = listBox1.Location;

    // Add two points to get a new location.
    shadowLocation = shadowLocation + new Size(5, 5);

    // Create a rectangleF. 
    RectangleF rectFToFill = 
        new RectangleF(shadowLocation, shadowSize);

    // Create a custom brush using a semi-transparent color, and 
    // then fill in the rectangle.
    Color customColor = Color.FromArgb(50, Color.Gray);
    SolidBrush shadowBrush = new SolidBrush(customColor);
    e.Graphics.FillRectangles(shadowBrush, new RectangleF[]{rectFToFill});

    // Dispose of the brush.
    shadowBrush.Dispose();
}