WPF: How to customize SelectionBoxItem in ComboBox

Question

I want to display a custom template/item as selected item in ComboBox (this item does not actually exist in the list of items and is updated differently). This does not even needs to be an item, just providing a custom view would work.

How can I do this while staying within current ComboBox theme (so no ControlTemplate replacement possible)? As far as I see, all of SelectionBox* properties are not editable and internally ComboBox uses unnamed ContentPresenter.

Answer 1

I would do it like this:

<Window.Resources>

  <DataTemplate x:Key="NormalItemTemplate" ...>
    ...
  </DataTemplate>

  <DataTemplate x:Key="SelectionBoxTemplate" ...>
    ...
  </DataTemplate>

  <DataTemplate x:Key="CombinedTemplate">
    <ContentPresenter x:Name="Presenter"
       Content="{Binding}"
       ContentTemplate="{StaticResource NormalItemTemplate}" />
    <DataTemplate.Triggers>
      <DataTrigger
        Binding="{Binding RelativeSource={RelativeSource FindAncestor,ComboBoxItem,1}}"
        Value="{x:Null}">
        <Setter TargetName="Presenter" Property="ContentTemplate"
                Value="{StaticResource SelectionBoxTemplate}" />
      </DataTrigger>
    </DataTemplate.Triggers>
  </DataTemplate>

</Window.Resources>

...

<ComboBox
  ItemTemplate="{StaticResource CombinedTemplate}"
  ItemsSource="..."
  ... />

The reason this works is that CombinedTemplate normally just uses NormalItemTemplate to present its data, but if there is no ComboBoxItem ancestor it assumes it is in the selection box so it uses SelectionBoxTemplate.

Note that the three DataTemplates could be included in any level of ResourceDictionary (not just at the Window level) or even directly within the ComboBox, depending on your preference.