What does a WPF DataGrid bind to the headers DataTemplate? - wpf

Question

When creating a WPF DataGrid, I have the option to set a DataTemplate to each column's HeaderTemplate, like this:
<DataGridTextColumn Binding="{Binding Name}">
<DataGridTextColumn.HeaderTemplate>
<DataTemplate>
<!-- just some random content, not important, but notice the bindings use RelativeSource-->
<StackPanel Orientation="Horizontal">
<TextBlock Text="Name" VerticalAlignment="Center"/>
<Button Command="{Binding Path=DataContext.SortFoldersByNameCommand, RelativeSource={RelativeSource AncestorType=DataGrid}}" Content="▲"
Foreground="{Binding Path=DataContext.IsFoldersSortByName, Converter={StaticResource EnabledToBrushConverter}, RelativeSource={RelativeSource AncestorType=DataGrid}}"/>
</StackPanel>
</DataTemplate>
</DataGridTextColumn.HeaderTemplate>
</DataGridTextColumn>
Now, notice that for my bindings to work inside that template, I need to set a RelativeSource, because this template is not bound to the grid's DataContext anymore.
The question is: what is bound to the header's DataTemplate?
If I simply try to put a <TextBlock Text="{Binding}"/>, for instance, the text is empty. I don't know what other kinds of tests I could do to find out.

Answer 1

The short answer is that the datacontext is nothing.
I threw a quick template into some datagrid I have on disk.
I used this thing to answer a few questions years back.
The header isn't in the visual tree and does not inherit datacontext.
My quick and dirty header template:
<DataGridTextColumn Binding="{Binding LastName}">
<DataGridTextColumn.HeaderTemplate>
<DataTemplate>
<TextBlock Text="XXXX"/>
</DataTemplate>
</DataGridTextColumn.HeaderTemplate>
</DataGridTextColumn>
Examine that textblock using Snoop...
Datacontext is empty:
This is why you do all that relativesource stuff to get to your property in the datacontext.
As an aside.
If you're doing much wpf development I recommend you give Snoop a go.
It's still way better than the in built thing.

Answer 2

Besides using Snoop (great tool!), you can also check that by adding a converter to the binding and inspecting it with a breakpoint. (The inspection is necessary if the binding is null, which is the case)
The Path=. indicates the current DataContext.
<DataGridTextColumn.HeaderTemplate>
<DataTemplate>
<TextBlock Text="{Binding Path=., Converter={local:TestConverter}}"/>
</DataTemplate>
</DataGridTextColumn.HeaderTemplate>
Where the converter is defined as:
class TestConverter : MarkupExtension, IValueConverter
{
//source to target
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value.GetType().FullName;
}
//target to source
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
public override object ProvideValue(IServiceProvider serviceProvider)
{
return this;
}
}