Why does the following xaml code not work? - xaml

Question

Why does this xaml code not work?
<Page
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
<Grid>
<Grid.Resources>
<ControlTemplate x:Key="btnTemplate" TargetType="{x:Type Button}">
<Grid>
<Ellipse Name="el1" Fill="Orange" Width="100" Height="100" />
<ContentPresenter VerticalAlignment="Center" HorizontalAlignment="Center" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Button.IsMouseOver" Value="True">
<Setter TargetName="el1" Property="Background" Value="Blue" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Grid.Resources>
<Button Content="Klick mich" Template="{StaticResource btnTemplate}"/>
</Grid>
</Page>

Answer 1

You are trying to set the Background property in your Trigger, but the Ellipse doesn't have a Background property. It has a Fill property. So you need to use:
<Grid>
<Grid.Resources>
<ControlTemplate x:Key="btnTemplate" TargetType="{x:Type Button}">
<Grid>
<Ellipse Name="el1" Fill="Orange" Width="100" Height="100" />
<ContentPresenter VerticalAlignment="Center" HorizontalAlignment="Center" />
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="Button.IsMouseOver" Value="True">
<Setter TargetName="el1" Property="Fill" Value="Blue" />
</Trigger>
</ControlTemplate.Triggers>
</ControlTemplate>
</Grid.Resources>
<Button Content="Klick mich" Template="{StaticResource btnTemplate}" />
</Grid>