Why do TargetType based Styles not work on custom template based controls?

Question

Style triggers are very handy and make our life very simple with WPF. However I am not able to utilize them when I have custom templates for controls.

For example this simple style in which I want All the TextBlock occurrences in my UserControl to collapse when they don't have any Text(null/empty) so that they do not occupy space in with in DataTemplate.

        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <Trigger Property="Text" Value="">
                    <Setter Property="Visibility" Value="Collapsed"/>
                </Trigger>
            </Style.Triggers>
        </Style>

This would work if my UserControl has simple elements like a series of TextBlocks and some other controls. But say, if I have a TreeView and I a ItemTemplateSelector to display TreeViewItem differently with various HierarchicalDataTemplate. For example one of them being this...

            <HierarchicalDataTemplate DataType="XYZ" ItemsSource ="{Binding}">
                <Grid x:Uid="Grid_7" Width="Auto" Height="Auto">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                    </Grid.ColumnDefinitions>
                    <Grid.RowDefinitions>
                        <RowDefinition/>
                    </Grid.RowDefinitions>
                   
                    <Image ... Grid.Column="0" Grid.Row="0" Margin="5,0,0,0"/>
                    <TextBlock ... Grid.Column="1" Grid.Row="0" Padding="5,0,0,0"/>
                    <TextBlock ... Grid.Column="2" Grid.Row="0" Foreground="Blue"Padding="5,0,0,0"/>
                    <TextBlock ... Grid.Column="3" Grid.Row="0" Foreground="Red"Padding="5,0,0,0"/>                  
                </Grid>
            </HierarchicalDataTemplate>

Now even if I add the style above under <TreeView.Resources> its never get applied. TextBlocks which have their Text as null/empty still occupy the space in the HierarchicalDataTemplat.

Can someone tell me, why doesn't the style gets applied to these HierarchicalDataTemplates? Is it because of design limitations? Or I am using it incorrectly?

Any help on this would be appreciated, thank you!

Answer 1

This is by design, as far as I know (if it's good or bad, that can be debatable; maybe things will change in future releases). For short, default styles do not work with objects directly derived from FrameworkElement, when placed inside FrameworkTemplate. See also http://shevaspace.blogspot.com/2007/03/wtf-of-wpf-part-one-templating-styling.html

Answer 2

Thanks Johnny, that calms my quest at least.

Answer 3

However, any solution to the original problem is welcome.