anim.xaml

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApplication1"
        mc:Ignorable="d"
        Title="MainWindow" Height="350" Width="525">
  <Window.Resources>
    <Storyboard x:Key="FadeIn" TargetName="TestButton">
      <DoubleAnimation Storyboard.TargetProperty="Opacity" From="0.0" To="1.0" Duration="0:0:1"/>
    </Storyboard>
    <Storyboard x:Key="FadeOut" TargetName="TestButton">
      <DoubleAnimation Storyboard.TargetProperty="Opacity" From="1.0" To="0.0" Duration="0:0:1"/>
    </Storyboard>
  </Window.Resources>
  <Grid>
    <Button x:Name="TestButton" Width="100" Height="30">
      HELLO
    </Button>
  </Grid>
</Window>