xaml - WPF ControlTemplate breaks style

Question

The stuff that does work

I need to style controls of a certain type that are children of a StackPanel. I'm using:

<StackPanel>
    <StackPanel.Resources>
        <Style TargetType="{x:Type TextBlock}">...</Style>
    </StackPanel.Resources>
    <TextBlock ...>
    ...
</StackPanel>

And this works fine! Each TextBlock looks to the resources of it's parent (the StackPanel) to find out how it should be styled. It doesn't matter how far down you nest the TextBlock down a StackPanel... if it doesn't find a style in its direct parent, it will look at its parent's parent and so on, until it finds something (in this case, the style that was defined in ).

The stuff that doesn't work

I ran into a problem when I nested a TextBlock inside a ContentControl, which had a Template (see code below). The ControlTemplate seems to disrupt the way a TextBlock retrieves its style from its parents, grandparents,...

The use of a ControlTemplate effectively seems to knock out cold the TextBlock's means of finding its rightful style (the one in StackPanel.Resources). When it encounters a ControlTemplate, it stops looking for its style in the resources up the tree, and instead defaults to the style in MergedDictionaries of the Application itself.

<StackPanel Orientation="Vertical" Background="LightGray">
    <StackPanel.Resources>
        <Style TargetType="{x:Type TextBlock}">
            <Setter Property="Foreground" Value="Green" />
        </Style>
    </StackPanel.Resources>

    <TextBlock Text="plain and simple in stackpanel, green" />
    <ContentControl>
        <TextBlock Text="inside ContentControl, still green" />
    </ContentControl>
    <ContentControl>
        <ContentControl.Template>
            <ControlTemplate TargetType="{x:Type ContentControl}">
                <StackPanel Orientation="Vertical">
                    <ContentPresenter />
                    <TextBlock Text="how come this one - placed in the template - is not green?" />
                </StackPanel>
            </ControlTemplate>
        </ContentControl.Template>
        <TextBlock Text="inside ContentControl with a template, this one is green as well" />
    </ContentControl>

</StackPanel>

Is there a way - besides duplicating the Style in StackPanel.Resources to ControlTemplate.Resources - to make the TextBlock inside that ControlTemplate find the defined style?

Thanks...

Answer 1

WPF considers ControlTemplates to be a boundry, and will not apply implicit styles (styles without an x:Key) inside of templates.

But there is one exception to this rule: anything that inherits from Control will apply implicit styles.

So you could use a Label instead of a TextBlock, and it would apply the implicit style defined further up your XAML hierarchy, however since TextBlock inherits from FrameworkElement instead of Control, it won't apply the implicit style automatically and you have to add it manually.

My most common way to get around this is to add an implicit style in the ControlTemplate.Resources that is BasedOn the existing implicit TextBlock style

    <ControlTemplate.Resources>
        <Style TargetType="{x:Type TextBlock}" 
               BasedOn="{StaticResource {x:Type TextBlock}}" />
    <ControlTemplate.Resources>

Other common ways of getting around this are:

• Place the implicit style in <Application.Resources>. Styles placed here will apply to your entire application, regardless of template boundaries. Be careful with this though, as it will apply the style to TextBlocks inside of other controls as well, like Buttons or ComboBoxes

  <Application.Resources>
      <Style TargetType="{x:Type TextBlock}">
          <Setter Property="Foreground" Value="Green" />
      </Style>
  </Application.Resources>
  

• Use a Label instead of a TextBlock since it's inherited from Control, so will apply implicit Styles defined outside the ControlTemplate

• Give the base style an x:Key and use it as the base style for an implicit TextBlock styles inside the ControlTemplate. It's pretty much the same as the top solution, however it's used for base styles that have an x:Key attribute

  <Style x:Key="BaseTextBlockStyle" TargetType="{x:Type TextBlock}">
      <Setter Property="Foreground" Value="Green" />
  </Style>
  
  ...
  
  <ControlTemplate.Resources>
      <Style TargetType="{x:Type TextBlock}" 
          BasedOn="{StaticResource BaseTextBlockStyle}" />
  <ControlTemplate.Resources>