Why can't I set the ItemTemplate?

Question

WPF

I write:

<DataTemplate x:Key="ClosableTabItemHeaderTemplate">
  <DataTemplate.Resources>
    <BooleanToVisibilityConverter x:Key="B2V"/>
  </DataTemplate.Resources>
  <Grid Background="Transparent" HorizontalAlignment="Stretch" MinWidth="100" ToolTip="{Binding}">
    <Grid.ColumnDefinitions>
      <ColumnDefinition/>
      <ColumnDefinition/>
    </Grid.ColumnDefinitions>

    <ContentPresenter Content="{Binding}" Grid.Column="0" HorizontalAlignment="Center"/>
    <Button Name="CLOSE"
            Command="{Binding Path=CloseCommand}" 
            Width="15" 
            Height="15" 
            Grid.Column="1" 
            Template="{StaticResource CloseButtonTemplate}"
            Visibility="{Binding IsSelected, RelativeSource={RelativeSource FindAncestor, AncestorType=TabItem},
            Converter={StaticResource B2V}}"/>
  </Grid>
  <DataTemplate.Triggers>
    <Trigger Property="IsMouseOver" Value="True">
      <Setter TargetName="CLOSE" Property="Visibility" Value="Visible"/>
    </Trigger>
  </DataTemplate.Triggers>
</DataTemplate>

<TabControl ItemTemplate="{StaticResource ClosableTabItemHeaderTemplate}">
  <TabControl.Items>
    <TabItem Header="This is test tab item with longer name." Width="100"/>
    <TabItem Header="Short name" Width="100"/>
  </TabControl.Items>
</TabControl>

But the pattern is not applied! What is the problem? :(

Answer 1

It seems to me that the problem is that you don't quite understand the difference between TabControl.ItemTemplate and TabItem.HeaderTemplate .
Judging by the template, you are trying to add a button to close the tab in the header header.
If so, then the following code should be used for this purpose:

<Grid>
        <Grid.Resources>
            <DataTemplate x:Key="ClosableTabItemHeaderTemplate">
                <DataTemplate.Resources>
                    <BooleanToVisibilityConverter x:Key="B2V"/>
                </DataTemplate.Resources>
                <Grid Background="Transparent" HorizontalAlignment="Stretch" MinWidth="100" ToolTip="{Binding}">
                    <Grid.ColumnDefinitions>
                        <ColumnDefinition/>
                        <ColumnDefinition/>
                    </Grid.ColumnDefinitions>

                    <ContentPresenter Content="{Binding}" Grid.Column="0" HorizontalAlignment="Center"/>
                    <Button Name="CLOSE"
            Command="{Binding Path=CloseCommand}" 
            Width="15" 
            Height="15" 
            Grid.Column="1" 
            Template="{StaticResource CloseButtonTemplate}"
            Visibility="{Binding IsSelected, RelativeSource={RelativeSource FindAncestor, AncestorType=TabItem},
            Converter={StaticResource B2V}}"/>
                </Grid>
                <DataTemplate.Triggers>
                    <Trigger Property="IsMouseOver" Value="True">
                        <Setter TargetName="CLOSE" Property="Visibility" Value="Visible"/>
                    </Trigger>
                </DataTemplate.Triggers>
            </DataTemplate>

            <Style TargetType="TabItem">
                <Setter Property="HeaderTemplate" Value="{StaticResource ClosableTabItemHeaderTemplate}" />                
            </Style>
            
        </Grid.Resources>

        <TabControl>
            <TabControl.Items>
                <TabItem Header="This is test tab item with longer name." Width="100"/>
                <TabItem Header="Short name" Width="100"/>
            </TabControl.Items>
        </TabControl>
    </Grid>