Why can't I use AlternationCount on a Border? - wpf

Question

In the code below if I change
<Style TargetType="{x:Type Border}">
To
<Style TargetType="{x:Type ListBoxItem}">
It will color my entire listboxitem the correct color. But I only want to hit the border background
<ListBox x:Name="FilteredMessagesListBox" BorderThickness="0" ScrollViewer.HorizontalScrollBarVisibility="Disabled" HorizontalContentAlignment="Stretch" SelectionMode="Extended" Background="Transparent" AlternationCount="2">
<ListBox.Resources>
<Style TargetType="{x:Type Border}">
<Style.Triggers>
<Trigger Property="ItemsControl.AlternationIndex" Value="0">
<Setter Property="Background" Value="LightBlue"></Setter>
</Trigger>
<Trigger Property="ItemsControl.AlternationIndex" Value="1">
<Setter Property="Background" Value="LightGreen"></Setter>
</Trigger>
</Style.Triggers>
</Style>
</ListBox.Resources>
<ListBox.ItemTemplate >
<DataTemplate>
<DockPanel Margin="0,0,0,3">
<Button x:Name="AttachmentImageButton" Click="AttachmentImageButton_Click" DockPanel.Dock="Bottom" MaxWidth="200" MaxHeight="200" HorizontalContentAlignment="Center" Visibility="{Binding ElementName=AttachmentImageButton, Converter={StaticResource cImageAttachmentToVisible}}" >
<Image Source="{Binding Path=Attachment}" x:Name="AttachmentImage" />
</Button>
<Button x:Name="AttachmentButton" Click="AttachmentButton_Click" DockPanel.Dock="Right" Visibility="{Binding ElementName=AttachmentButton, Converter={StaticResource cAttachmentToVisible}}" >
<Image Source="/MobilWPF;component/Resources/Images/PaperClip/PaperClip.jpg" Width="20" Height="20"/>
</Button>
<TextBlock Text="{Binding Converter={StaticResource cGetInstantMessageHeader}}" Width="120" TextWrapping="Wrap" HorizontalAlignment="Left" Background="Transparent" FontSize="10" DockPanel.Dock="Left"/>
<Border DockPanel.Dock="Left" HorizontalAlignment="Stretch" CornerRadius="5">
<DockPanel>
<TextBlock Margin="5,0,0,0" Text="{Binding Path=Message}" TextWrapping="Wrap" DockPanel.Dock="Left" HorizontalAlignment="Left" Background="Transparent" FontSize="12"/>
</DockPanel>
</Border>
</DockPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
I tried this:
<Border DockPanel.Dock="Left" HorizontalAlignment="Stretch" CornerRadius="5">
<Border.Triggers>
<DataTrigger Binding="{Binding Path=(ItemsControl.AlternationIndex), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}" Value="0">
<Setter Property="Background" Value="LightBlue"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=(ItemsControl.AlternationIndex), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}" Value="1">
<Setter Property="Background" Value="LightGreen"/>
</DataTrigger>
</Border.Triggers>
<DockPanel>
<TextBlock Margin="5,0,0,0" Text="{Binding Path=Message}" TextWrapping="Wrap" DockPanel.Dock="Left" HorizontalAlignment="Left" Background="Transparent" FontSize="12"/>
</DockPanel>
</Border>
Cannot find the static member 'BackgroundProperty' on the type 'ContentPresenter'

Answer 1

The problem is that AlternationIndex is set on the containers (ListBoxItems in this case), not on some child within them. You could solve this by using DataTriggers that bind to AlternationIndex on the parent ListBoxItem instead:
<DataTrigger Binding="{Binding Path=(ItemsControl.AlternationIndex), RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type ListBoxItem}}}" Value="0">
<Setter Property="Background" Value="LightBlue"/>
</DataTrigger>