Something

   public class R : Shape {
        protected override void OnPropertyChanged (DependencyPropertyChangedEventArgs e)
        {
            Console.WriteLine ("here");
        }

        protected override Geometry DefiningGeometry
        {
            get {
                return new LineGeometry ();
            }
        }
    }

    public partial class Window1 : System.Windows.Window
    {

        public  Window1 ()
        {
            InitializeComponent ();

            R r = new R ();
            R r2 = new R ();
            RotateTransform rr = new RotateTransform ();
            r.RenderTransform = rr;
            r2.RenderTransform = rr;
            rr.Angle = 10;

        }