WPF GroupBox ControlTemplate: how to apply style only to elements in the header?

Question

I am trying to make ControlTemplatefor GroupBoxsuch that if a is TextBlockdeclared in Header, it Backgroundshould be set to yellow.

The problem is that although I am defining a style for TextBlockin ContentPresenterfor Header, it does not apply except for those TextBlockthat WPF auto-generates.

Here is the code:

<Window
  x:Class="TestHeaderTemplate.Window1"
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  Title="Window1"
  SizeToContent="WidthAndHeight">
  <Window.Resources>    
    <Style
      TargetType="{x:Type GroupBox}">
      <Setter
        Property="Template">
        <Setter.Value>
          <ControlTemplate
            TargetType="{x:Type GroupBox}">            
            <Border
              Margin="{TemplateBinding Margin}"
              BorderBrush="Black"
              BorderThickness="1">
              <StackPanel>
                <Border
                  Margin="0,0,0,5"
                  BorderThickness="5"
                  BorderBrush="LightBlue"
                  >
                  <ContentPresenter
                    ContentSource="Header">
                    <ContentPresenter.Resources>
                      <Style
                        TargetType="{x:Type TextBlock}">
                        <Setter
                          Property="Background"
                          Value="Yellow" />
                      </Style>
                    </ContentPresenter.Resources>
                  </ContentPresenter>
                </Border>
                <ContentPresenter
                  ContentSource="Content" />
              </StackPanel>
            </Border>
          </ControlTemplate>
        </Setter.Value>
      </Setter>
    </Style>
  </Window.Resources>
  <StackPanel>
    <TextBox
      Text="All TextBoxes in a GroupBox Header should be yellow, whether declared or autogenerated." />
    <GroupBox
      x:Name="firstGroupBox"
      Margin="5"
      Header="I am a TextBlock autogenerated by WPF. Since I'm in the Header, I should be yellow.">
        <TextBlock
          Text="I'm a TextBlock declared in the content of the GroupBox. I should NOT be yellow." />      
    </GroupBox>
    <GroupBox
      x:Name="secondGroupbox"
      Margin="5"
      >
      <HeaderedContentControl.Header>      
          <TextBlock
            x:Name="notStyledTextBlock"
            Text="I'm a TextBlock declared in the header. I should be yellow since I'm in the header."
            VerticalAlignment="Center" />          
      </HeaderedContentControl.Header>
      <TextBlock
        Text="I'm declared in the content so I should not be yellow." />
    </GroupBox>
  </StackPanel>
</Window>

As you can see, if you try, the background TextBlockwith the name notStyledTextBlockin the second is GroupBoxnot yellow, which means the style defined in the resources ContentPresenteris ControlTemplatenot applied.

Surprisingly, the one that WPF autogenerated as a container for the title text of the first GroupBoxhas a yellow background color.

, notStyledTextBlock TextBlock?

Answer 1

GroupBoxes ContentPresenter. , , . , , ( : , , ).