public partial class Window1 : Window
    {
        private Storyboard myStoryBoard; 

        public Window1()
        {
            InitializeComponent();

            this.RegisterName("TestButton1", TestButton1);
            DoubleAnimation da = new DoubleAnimation();
            da.From = 1.0;
            da.To = 0.0;
            myStoryBoard = new Storyboard();
            myStoryBoard.Children.Add(da);
            Storyboard.SetTargetName(da, "TestButton1");
            Storyboard.SetTargetProperty(da, new PropertyPath(Button.OpacityProperty));
        }

        private void TestButton_Click(object sender, RoutedEventArgs e)
        {
            myStoryBoard.Begin(this, true);
        }
    }

------解决方案--------------------

我这么试的一个，在后台调用好使，你试着改改。
挺长时间不用，有点忘了。
txttt.Begin();
C# code

<Rectangle Name="MyRectangle" Width="100" Height="100">
            <Rectangle.Fill>
                <SolidColorBrush x:Name="MySolidColorBrush" Color="Blue" />
            </Rectangle.Fill>
            <Rectangle.Triggers>
                <EventTrigger RoutedEvent="Page.Loaded">
                    <BeginStoryboard>
                        <Storyboard x:Name="txttt" RepeatBehavior="Forever" AutoReverse="True">
                            <DoubleAnimation 
                                Storyboard.TargetName="MyRectangle"
                                Storyboard.TargetProperty="Width"
                                From="100" To="200" Duration="0:0:1" />
                                            <ColorAnimation 
                                Storyboard.TargetName="MySolidColorBrush"
                                Storyboard.TargetProperty="Color"
                                From="Blue" To="Red" Duration="0:0:1" />
                        </Storyboard>
                    </BeginStoryboard>
                </EventTrigger>
            </Rectangle.Triggers>
        </Rectangle>