class MyPathClass
{
    public static readonly DependencyProperty MyShapeProperty = DependencyProperty.RegisterAttached("MyShape", typeof(Geometry), typeof(MyPathClass), new FrameworkPropertyMetadata(OnMyShapeChanged()));


    private static void OnMyShapeChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        (MyPathClass)d.MyShape = e.NewValue;
    }


    public Geometry MyShape
    {
        get
        {
            return (Geometry)GetValue(MyShapeProperty);
        }
        set
        {
            //Вот сюда без OnMyShapeChanged отказывается заходить
            SetValue(MyShapeProperty, Value);
        }
    }

}