I was writing some WPF testing code today. It was extremely simple code which just programmatically creates a Rectangle and adds it to a canvas. The code this something like this: Rectangle rectangle = new Rectangle { Fill = new SolidColorBrush(Colors.Blue), Stroke = new SolidColorBrush(Colors.Blue), Width = 150, Height = 120 }; rectangle.SetValue(Canvas.L... 100); rectangle.SetValue(Canvas.T... 100); canvas.Children.Add(rectang... However, I got an exception on rectangle.SetValue(Canvas.L...