Using the same ControlTemplate in different styles and override a property - wpf

Question

For two TabItems I have two Styles, both use the same ControlTemplate. Now I want styleTabB to show a yellow underline instead of green, but still using the ControlTemplate. How can I modify the Style to accomplish this?
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:WpfApplication1"
mc:Ignorable="d"
Title="MainWindow" Height="350" Width="525" FontSize="16">
<Window.Resources>
<ControlTemplate x:Key="ctrlTemplate" TargetType="{x:Type TabItem}">
<Grid>
<TextBlock Name="tbTabItemHeaderText"
Text="{TemplateBinding Header}"
Grid.Column="0"
Background="Thistle"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Margin="3,0,0,3">
<TextBlock.TextDecorations>
<TextDecorationCollection>
<TextDecoration Location="Underline"
PenThicknessUnit="Pixel"
PenOffsetUnit="Pixel"
PenOffset="2">
<TextDecoration.Pen>
<Pen Brush="Green" Thickness="4" />
</TextDecoration.Pen>
</TextDecoration>
</TextDecorationCollection>
</TextBlock.TextDecorations>
</TextBlock>
</Grid>
</ControlTemplate>
<!-- Style Tab A -->
<Style x:Key="styleTabA" TargetType="{x:Type TabItem}">
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
<!-- Style Tab B -->
<Style x:Key="styleTabB" TargetType="{x:Type TabItem}">
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
</Window.Resources>
<Grid>
<TabControl Name="tabControl">
<TabItem Name="tabItem_1" Header="--- Tab A ---" Style="{StaticResource styleTabA}"/>
<TabItem Name="tabItem_2" Header="--- Tab B ---" Style="{StaticResource styleTabB}" />
</TabControl>
</Grid>
</Window>
UPDATE
I tried the proposal of Chris W., but no underline at all is shown:
<ControlTemplate x:Key="ctrlTemplate" TargetType="{x:Type TabItem}">
...
<TextDecoration.Pen>
<Pen Brush="{TemplateBinding Tag}" Thickness="4" />
</TextDecoration.Pen>
...
</ControlTemplate>
<!--Style Tab A-->
<Style x:Key="styleTabA" TargetType="{x:Type TabItem}">
<Setter Property="Tag" Value="Green" />
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
<!--Style Tab B-->
<Style x:Key="styleTabB" TargetType="{x:Type TabItem}">
<Setter Property="Tag" Value="Yellow" />
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>

Answer 1

You can set the style of the children within the child by targetting particular types. Here all Pen are updated with Yellow color.
<Style x:Key="styleTabB" TargetType="{x:Type TabItem}">
<Style.Resources>
<Style TargetType="{x:Type Pen}">
<Setter Property="Brush" Value="Yellow"></Setter>
</Style>
</Style.Resources>
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>

Answer 2

#CarbineCoder is correct for most instances but for your instance you're right and the error you received from his would be expected since Pen isn't a TargetType. However if we tweak it just a little to hit the actual FrameworkElement of which TextDecorations is a property let's try this...and read the whole thing since the first snippet is just an example explanation.
<Style x:Key="styleTabB" TargetType="{x:Type TabItem}">
<Style.Resources>
<Style TargetType="{x:Type TextBlock}">
<Setter Property="TextDecorations">
<Setter.Value>
<TextDecorationCollection>
<TextDecoration>
<TextDecoration.Pen>
<Pen Brush="Yellow"/>
</TextDecoration.Pen>
</TextDecoration>
</TextDecorationCollection>
</Setter.Value>
</Setter>
</Style>
</Style.Resources>
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
** *Except that's not going to work because you're hoping for something that you've already provided an explicit property value for to inherit from parent, and it doesn't work that way. How about instead we bring in the handy Tag property (which I abuse all the time) to piggy back in our value and allow a way to talk to our buddy on the inside of that bugger by making some quick edits to your ControlTemplate like;
<!-- In your STYLE Template you would want to add a default setter of;
<Setter Property="Tag" Value="Green"/>
-->
<ControlTemplate x:Key="ctrlTemplate" TargetType="{x:Type TabItem}">
<Grid Margin="0,0,0,0">
<TextBlock Name="_tbTabItemHeaderText"
Text="{TemplateBinding Header}"
Grid.Column="0"
Background="Thistle"
VerticalAlignment="Center"
Margin="3,0,0,3">
<TextBlock.TextDecorations>
<TextDecorationCollection>
<TextDecoration Location="Underline"
PenThicknessUnit="Pixel"
PenOffsetUnit="Pixel"
PenOffset="2">
<TextDecoration.Pen>
<Pen Brush="{TemplateBinding Tag}" Thickness="4" />
</TextDecoration.Pen>
</TextDecoration>
</TextDecorationCollection>
</TextBlock.TextDecorations>
</TextBlock>
</Grid>
</ControlTemplate>
Now we should be able to hit like;
<!-- Style Tab A
: This guy should just keep it green IF you applied the default setter mentioned above to the STYLE template -->
<Style x:Key="styleTabA" TargetType="{x:Type TabItem}">
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
<!-- Style Tab B
: This guy should turn it Yellow -->
<Style x:Key="styleTabB" TargetType="{x:Type TabItem}">
<Setter Property="Tag" Value="Yellow"/>
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
I didn't have time to test, but it seems like this should work fine for your scenario. Hope it helps.

Answer 3

I made a little change on Chris W.'s solution and now it's working:
<ControlTemplate x:Key="ctrlTemplate" TargetType="{x:Type TabItem}">
...
<TextDecoration.Pen>
<!--Changed next line from <Pen Brush="{TemplateBinding Tag}" Thickness="4" /> to:-->
<Pen Brush="{Binding Path=Tag, RelativeSource={RelativeSource TemplatedParent}}" Thickness="4" />
</TextDecoration.Pen>
...
</ControlTemplate>
<!--Style Tab A-->
<Style x:Key="styleTabA" TargetType="{x:Type TabItem}">
<Setter Property="Tag" Value="Green" />
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>
<!--Style Tab B-->
<Style x:Key="styleTabB" TargetType="{x:Type TabItem}">
<Setter Property="Tag" Value="Yellow" />
<Setter Property="Template" Value="{StaticResource ctrlTemplate}" />
</Style>