protected override void OnRender(DrawingContext drawingContext)
{
  // Get a rectangle that represents the desired size of the rendered element
  // after the rendering pass.  This will be used to draw at the corners of the 
  // adorned element.
  Rect adornedElementRect = new Rect(this.AdornedElement.RenderSize);

  // Some arbitrary drawing implements.
  SolidColorBrush renderBrush = new SolidColorBrush(Colors.Green);
  renderBrush.Opacity = 0.2;
  Pen renderPen = new Pen(new SolidColorBrush(Colors.Navy), 1.5);
  double renderRadius = 5.0;

  // Just draw a circle at each corner.
  drawingContext.DrawEllipse(renderBrush, renderPen, adornedElementRect.TopLeft, renderRadius, renderRadius);
  drawingContext.DrawEllipse(renderBrush, renderPen, adornedElementRect.TopRight, renderRadius, renderRadius);
  drawingContext.DrawEllipse(renderBrush, renderPen, adornedElementRect.BottomLeft, renderRadius, renderRadius);
  drawingContext.DrawEllipse(renderBrush, renderPen, adornedElementRect.BottomRight, renderRadius, renderRadius);
}