Question

I am trying to make a custom style for a TabItem Header. I got it to work by accident.

this fails:

<Style TargetType="{x:Type TabItem}" x:Name="TabHeader3" x:Key="test"> 

but this works

<Style TargetType="{x:Type TabItem}" x:Name="TabHeader3"> 

What’s going on?

Answer 1

The first Style you have defined is an “explicit” Style, so you must explicitly use it like so:

<TabItem Style="{StaticResource test}" />

The second Style you have defined is an “implicit” Style. So it will be applied to all TabItem controls below it in the visual/logical tree, or to all TabItem controls if it’s defined in the application resources.

Your second Style is equivalent to:

<Style TargetType="{x:Type TabItem}" x:Name="TabHeader3" x:Key="{x:Type TabItem}">

So the key is the Type of the object to which it should be applied.

If a TabItem has a Style explicitly defined (like I show above), then any implicit Styles will not be used. Also, if you have two implicit Styles defined, then the closest one wins. So here:

<Window>
    <Window.Resources>
        <Style TargetType="{x:Type TabItem}"> 
            <Setter Property="Background" Value="Red" />
        </Style>
    </Window.Resources>
    <Grid>
        <Grid.Resources>
             <Style TargetType="{x:Type TabItem}"> 
                 <Setter Property="Background" Value="Blue" />
            </Style>
        </Grid.Resources>
        ...
        <TabItem ... />
        ...
    </Grid>
</Window>

The Blue Style will take precedence over the Red Style.

Finally, you generally don’t need to include x:Name on your Styles.