Why a Listbox DataTemplate does not use the Windows.Resources styles?

Question

I have the following XAML (simplified):

<Window ...

    <Window.Resources>
        <Style TargetType="{x:Type TextBlock}" >
            <Setter Property="FontSize" Value="28" />
            <Setter Property="Margin" Value="3" />
            <Setter Property="Foreground" Value="Green" />
        </Style>
    </Window.Resources>

    <StackPanel>

        <ListBox ItemsSource=...
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <StackPanel Orientation="Horizontal" >
                        <TextBlock Text="{Binding Index}" />
                        <TextBlock Text="-" />
                        <TextBlock Text="{Binding Hours, StringFormat={}{0:00}}" />
                        <TextBlock Text=":" />
                        <TextBlock Text="{Binding Minutes, StringFormat={}{0:00}}" />
                        <TextBlock Text=":" />
                        <TextBlock Text="{Binding Seconds, StringFormat={}{0:00}}" />
                    </StackPanel>
                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

        ...

With this code the Style defined in the Window.Resources is not being applied to the TextBlock inside the DataTemplate but it is on other TextBlocks on the Window.

If I copy the Style and set it in the DataTemplate resources like this:

        <DataTemplate.Resources>
            <Style TargetType="{x:Type TextBlock}" >
                <Setter Property="FontSize" Value="28" />
                <Setter Property="Margin" Value="3" />
                <Setter Property="Foreground" Value="Green" />
            </Style>
        </DataTemplate.Resources>

Then it works. Any idea why do I need to duplicate the style?

Thanks in advance.

Answer 1

Implicit Styles will apply in templates only to types that inherit from System.Windows.Controls.Control and since TextBlock inherits directly from System.Windows.FrameworkElement it won't work. You have to either give your style x:Key and use it explicitly or declare your style in Application.Resources but then it will apply to all TextBlocks and by this I mean basically every displayed bit of text, in whole application

Answer 2

It's a WPF quirk. When a control doesn't inherit from Control, but directly from FrameworkElement, implicit style lookup inside a template skips directly to the application resources. If you put the style in the application resources (App.xaml) it would work.

Alternatively, you can use a named resource and BasedOn to reference it:

    <DataTemplate.Resources>
        <Style TargetType="TextBlock" BasedOn="{StaticResource MyTextStyle}" />
    </DataTemplate.Resources>