public class myline : Shape
    {
        LineGeometry linegeo = new LineGeometry();

        // Dependency properties
        public static readonly DependencyProperty StartPointProperty =
            LineGeometry.StartPointProperty.AddOwner(
                typeof(myline),
                new FrameworkPropertyMetadata(new Point(0, 0),
                    FrameworkPropertyMetadataOptions.AffectsMeasure));

        public static readonly DependencyProperty EndPointProperty =
            LineGeometry.EndPointProperty.AddOwner(
                typeof(myline),
                new FrameworkPropertyMetadata(new Point(0, 0),
                    FrameworkPropertyMetadataOptions.AffectsMeasure));

        public Point StartPoint
        {
            set { SetValue(StartPointProperty, value); }
            get { return (Point)GetValue(StartPointProperty); }
        }

        public Point EndPoint
        {
            set { SetValue(EndPointProperty, value); }
            get { return (Point)GetValue(EndPointProperty); }
        }

        // Required DefiningGeometry override
        protected override Geometry DefiningGeometry
        {
            get
            {
                linegeo.StartPoint = StartPoint;
                linegeo.EndPoint = EndPoint;
                return linegeo;
            }
        }
   

    }

         myline my = new myline();
            my.StartPoint = new Point(0, 0);
            my.EndPoint = new Point(99, 99);
            my.Stroke = Brushes.Black;
            canvas2.Children.Add(my);
double n=Canvas.GetLeft(my);//возвращает nan.почему?