ListBox items separator when more than one item - wpf

Question

I have a ListBox and I'm trying to insert a separator between items but I want the separator to show when there is more than one item.
<ListBox Grid.Row="1" x:Name="WarningListBox" Visibility="Visible"
HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch"
ItemsSource="{Binding Path=Warnings}">
<ListBox.Style>
<Style TargetType="{x:Type ListBox}">
<Setter Property="Height" Value="0"/>
<Style.Triggers>
<DataTrigger Binding="{Binding CanShowAll}" Value="True">
<Setter Property="Height" Value="Auto"/>
<Setter Property="MaxHeight" Value="120"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ListBox.Style>
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem" BasedOn="{StaticResource NewDesignListItemBoxStyle}">
<Setter Property="Margin" Value="0"/>
<Setter Property="Padding" Value="0"/>
</Style>
</ListBox.ItemContainerStyle>
<ListBox.Resources>
<DataTemplate x:Key="TypeOneViewModel" DataType="{x:Type vm:TypeOneViewModel}">
<notifications:TypeOneNotification Focusable="False" MinHeight="30" Background="Beige"/>
</DataTemplate>
<DataTemplate x:Key="TypeTwoViewModel" DataType="{x:Type vm:TypeTwoViewModel}">
<notifications:TypeTwoNotification Focusable="False" MinHeight="30" Background="Beige"/>
</DataTemplate>
</ListBox.Resources>
<ListBox.ItemTemplateSelector>
<templateSelectors:WarningTemplateSelector
TypeOneDataTemplate="{StaticResource TypeOneViewModel}"
TypeTwoDataTemplate="{StaticResource TypeTwoViewModel}"/>
</ListBox.ItemTemplateSelector>
</ListBox>
I've tried adding the Separator right before the closing tag
<Separator Margin="30 0 30 0" BorderBrush="#CCCCCC" BorderThickness="2"/>
but it crashes the application. Should I somehow put it in a DataTemplate? I think it will then show under each item (even when there's only one item).

Answer 1

I think, you want to achieve something like this:
<DataTemplate x:Key="TypeOneViewModel" DataType="{x:Type vm:TypeOneViewModel}">
<DockPanel LastChildFill="True">
<Separator x:Name="separator" HorizontalAlignment="Stretch" VerticalAlignment="Top" DockPanel.Dock="Top">
<Separator.Style>
<Style TargetType="Separator">
<Setter Property="Visibility" Value="Visible"/>
<Style.Triggers>
<DataTrigger Binding="{Binding RelativeSource={RelativeSource PreviousData}, FallbackValue={x:Null}}" Value="{x:Null}">
<Setter Property="Visibility" Value="Collapsed"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Separator.Style>
</Separator>
<!--Rest of your DataTemplate-->
<notifications:TypeOneNotification Focusable="False" MinHeight="30" Background="Beige"/>
</DockPanel>
</DataTemplate>
This will hide the separator, if item have no PreviousData, so for the 1st item.
You can easy take Separator style outside if used more then once, and just call it as StaticResource.