从c#设置线段动画不起作用

这是我认为正确的代码。但它不起作用。这是某种错误还是我做错了什么？从xaml设置线段动画效果很好。

主窗口.xaml:

<Window x:Class="XXX.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">
<Canvas Name="canvas">
</Canvas>
</Window>

主窗口.xaml.cs:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        anim();
    }
    void anim()
    {
        Path path = new Path { Stroke = Brushes.Red, StrokeThickness = 1 };
        PathGeometry pg = new PathGeometry();
        PathFigureCollection pfc = new PathFigureCollection();
        PathFigure pf = new PathFigure { StartPoint = new Point(50, 50) };
        Storyboard sb = new Storyboard 
        { 
            Duration = System.Windows.Duration.Forever, 
            BeginTime = new TimeSpan(0, 0, 0), 
            RepeatBehavior = System.Windows.Media.Animation.RepeatBehavior.Forever, 
            Name = "sb" 
        };
        LineSegment ls = new LineSegment 
        { 
            IsSmoothJoin = true, 
            Point = new Point(80, 50) 
        };
        PointAnimation pa = new PointAnimation
        {
            BeginTime = new TimeSpan(0, 0, 0, 0, 0),
            Duration = new Duration(new TimeSpan(0, 0, 0, 1)),
            From = new Point(0, 0),
            To = new Point(0, 100),
            RepeatBehavior = System.Windows.Media.Animation.RepeatBehavior.Forever,
            AutoReverse = true,
        };
        Storyboard.SetTarget(pa, ls);
        Storyboard.SetTargetProperty(pa, new PropertyPath("Point"));
        sb.Children.Add(pa);
        pf.Segments.Add(ls);
        pfc.Add(pf);
        pg.Figures = pfc;
        path.Data = pg;
        canvas.Children.Add(path);
        sb.Begin();

    }
}

对我来说，它看起来像某人Begin（）；未激发或故事板。settarge/setproperty设置不正确。但如果真是这样，又有什么错呢？

int count = 0;
        foreach(var f in pf.Segments)
        {                
            f.BeginAnimation(LineSegment.PointProperty, (PointAnimation)sb.Children.ElementAt(count));
            count++;
        }