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<Window xmlns:WpfApplication1="clr-namespace:WpfApplication1"  x:Class="WpfApplication1.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">
  <Window.Resources>
    <ResourceDictionary
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:local="clr-namespace:WpfApplication1">
      <Style TargetType="{x:Type local:ExpanderEx}" x:Key="ExpanderExStyle">
        <Setter Property="Foreground" Value="{DynamicResource {x:Static SystemColors.ControlTextBrushKey}}"/>
        <Setter Property="Background" Value="Transparent"/>
        <Setter Property="HorizontalContentAlignment" Value="Stretch"/>
        <Setter Property="VerticalContentAlignment" Value="Stretch"/>
        <Setter Property="BorderBrush" Value="Transparent"/>
        <Setter Property="BorderThickness" Value="1"/>
        <Setter Property="Template">
          <Setter.Value>
            <ControlTemplate TargetType="{x:Type local:ExpanderEx}">
              <Border SnapsToDevicePixels="true" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" CornerRadius="3">
                <DockPanel>
                  <ContentControl
                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                    Margin="{TemplateBinding Padding}"
                    x:Name="HeaderSite"
                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                    DockPanel.Dock="Top"
                    Content="{TemplateBinding Header}"/>
                  <ContentPresenter
                    Focusable="false"
                    Visibility="Collapsed"
                    HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
                    Margin="{TemplateBinding Padding}"
                    x:Name="ExpandSite"
                    VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
                    DockPanel.Dock="Bottom"/>
                </DockPanel>
              </Border>
              <ControlTemplate.Triggers>
                <Trigger Property="IsMouseDoubleClicked" Value="True">
                  
                  <Setter Property="Visibility" TargetName="ExpandSite" Value="Visible"/>
                  <Setter Property="IsExpanded" Value="True"/>
                </Trigger>
              </ControlTemplate.Triggers>
            </ControlTemplate>
          </Setter.Value>
        </Setter>
      </Style>
    </ResourceDictionary>


  </Window.Resources>
  <Grid>
    <ListBox>
      <ListBox.Items>
        <ListBoxItem>
          <WpfApplication1:ExpanderEx Style="{DynamicResource ExpanderExStyle}" Header="Mein Expander" IsExpanded="True">
            <TextBlock Text="Foobar" />
          </WpfApplication1:ExpanderEx>
        </ListBoxItem>
        <ListBoxItem>
          <TextBlock Text="Drunter!" />
        </ListBoxItem>
      </ListBox.Items>
    </ListBox>


  </Grid>
</Window>