Can't set both ContentTemplateSelector and Template properties on a DataGridColumnHeader

Question

In short, the question title says it all. For those that want more detail, here is the crux of my problem: I need to apply a custom ControlTemplate to the DataGridColumnHeader elements in my DataGrid control, but I also need to style them differently, depending on the cell data nearest the header. However, when I set both the ContentTemplateSelector and Template properties on a DataGridColumnHeader element, the DataTemplateSelector that is set as the value of the ContentTemplateSelector property is not called. Commenting out the Template property setting confirms this to be the case, as the DataTemplateSelector element will now be called.

Yes, I know that you guys love to see some code, but I have completely templated the whole DataGrid control to look like Excel, so as you can imagine, I have far too much code to display here. But just to please you code hungry devs, I've recreated my problem in a much simpler example... let's first see the XAML:

<Window x:Class="WpfApp1.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:WpfApp1"
    xmlns:System="clr-namespace:System;assembly=mscorlib"
    mc:Ignorable="d"
    Title="MainWindow" Height="350" Width="525">
    <Grid>
        <DataGrid>
            <DataGrid.Columns>
                <DataGridTemplateColumn>
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <TextBlock Text="{Binding}" />
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
            <DataGrid.Items>
                <System:String>One</System:String>
                <System:String>Two</System:String>
                <System:String>Three</System:String>
            </DataGrid.Items>
            <DataGrid.Resources>
                <Local:StringDataTemplateSelector x:Key="StringDataTemplateSelector" />
                <Style TargetType="{x:Type DataGridColumnHeader}" BasedOn="{StaticResource {x:Type DataGridColumnHeader}}">
                    <Setter Property="ContentTemplateSelector" Value="{StaticResource StringDataTemplateSelector}" />
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
                                <Grid>
                                    <Thumb x:Name="PART_LeftHeaderGripper" HorizontalAlignment="Left" />
                                    <Thumb x:Name="PART_RightHeaderGripper" HorizontalAlignment="Right" />
                                </Grid>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Style>
            </DataGrid.Resources>
        </DataGrid>
    </Grid>
</Window>

Now the most simple DataTemplateSelector class:

using System.Diagnostics;
using System.Windows;
using System.Windows.Controls;

namespace WpfApp1
{
    public class StringDataTemplateSelector : DataTemplateSelector
    {
        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            Debugger.Break();
            return null;
        }
    }
}

In the XAML, we see a DataGrid, with just one DataGridTemplateColumn and three string values, one on each row, and some resources. There is a Style for the DataGridColumnHeader element in the Resource section, with the most simple ControlTemplate set up for it, that only includes the required named parts from the default ControlTemplate.

If you run the application as it is, then it will NOT currently break at the Debugger.Break() method in the StringDataTemplateSelector class. This is unexpected. If you now comment out the setting of the Template property in the Style and run the application again, then you will now see that program execution will now break at the Debugger.Break() method, as expected.

Further information:

In the Remarks section of the ContentControl.ContentTemplateSelector Property page of MSDN, it states that

If both the ContentTemplateSelector and the ContentTemplate properties are set, then this property is ignored.

However, it does not mention the Template property and there is also no mention of this on the Control.Template Property page on MSDN.

Furthermore, I tried this same setup using a simple Button control and can confirm that setting both the ContentTemplateSelector and the ContentTemplate properties on that does NOT stop the StringDataTemplateSelector class from being called:

<ItemsControl>
    <ItemsControl.Resources>
        <Local:StringDataTemplateSelector x:Key="StringDataTemplateSelector" />
        <Style TargetType="{x:Type Button}">
            <Setter Property="ContentTemplateSelector" Value="{StaticResource StringDataTemplateSelector}" />
            <Setter Property="Template">
                <Setter.Value>
                    <ControlTemplate TargetType="{x:Type Button}">
                        <Grid>
                            <Ellipse Stroke="Red" StrokeThickness="1" Width="{TemplateBinding ActualWidth}" Height="{TemplateBinding Height}" />
                            <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center" />
                        </Grid>
                    </ControlTemplate>
                </Setter.Value>
            </Setter>
        </Style>
    </ItemsControl.Resources>
    <Button Content="One" />
    <Button Content="Two" />
    <Button Content="Three" />
</ItemsControl>

So, what I'm after is a way to apply a custom ControlTemplate element to the DataGridColumnHeader objects, yet still be able to have the DataTemplateSelector class called during the rendering process.

Answer 1

add a content presenter in your controltemplate?

<ControlTemplate TargetType="{x:Type DataGridColumnHeader}">
    <Grid>
           <Thumb x:Name="PART_LeftHeaderGripper" HorizontalAlignment="Left" />
           <Thumb x:Name="PART_RightHeaderGripper" HorizontalAlignment="Right" />
           <ContentPresenter></ContentPresenter>
    </Grid>
</ControlTemplate>