Page.xaml

<UserControl x:Class="doubleanimation.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300">
    <StackPanel>
        <TextBlock Margin="10" TextWrapping="Wrap">This sample demonstrates how to use DoubleAnimation to create a rectangle that fades in and out of view after it is loaded.</TextBlock>
    <!-- <SnippetDoubleanimation> -->
    <StackPanel>
        <StackPanel.Resources>
            <Storyboard x:Name="myStoryboard">
                <DoubleAnimation
              Storyboard.TargetName="MyAnimatedRectangle"
              Storyboard.TargetProperty="Opacity"
              From="1.0" To="0.0" Duration="0:0:3"
              AutoReverse="True" RepeatBehavior="Forever" />
            </Storyboard>
        </StackPanel.Resources>

        <Rectangle Loaded="Start_Animation" x:Name="MyAnimatedRectangle"
         Width="100" Height="100" Fill="Blue" />

    </StackPanel>
    <!-- </SnippetDoubleanimation> -->
    </StackPanel>
</UserControl>