DataGridColumnHeader not applied when default style present

[brinko99]

I've been trying to figure out why a custom DataGridColumnHeader style will not be applied when a default header style is also present. Any help is appreciated.

Per the example below, when I apply the DataGridTallStyle to a DataGrid, the DataGridColumnHeaderTallStyle is not applied.

If DataGridColumnHeaderTallStyle is applied directly to the DataGrid it works as expected. The same situation with rows works in all cases.

See the attached project for a repo.

Thanks

<Style x:Key="{x:Type DataGridRow}" TargetType="{x:Type DataGridRow}">
    <Setter Property="MinHeight" Value="28" />
</Style>

<Style x:Key="DataGridRowTallStyle" TargetType="{x:Type DataGridRow}" BasedOn="{StaticResource {x:Type DataGridRow}}">
    <Setter Property="MinHeight" Value="55" />
</Style>

<Style x:Key="{x:Type DataGridColumnHeader}" TargetType="{x:Type DataGridColumnHeader}">
    <Setter Property="MinHeight" Value="28" />
</Style>

<Style x:Key="DataGridColumnHeaderTallStyle" TargetType="DataGridColumnHeader" BasedOn="{StaticResource {x:Type DataGridColumnHeader}}">
    <Setter Property="MinHeight" Value="55" />
</Style>

<Style x:Key="DataGridTallStyle" TargetType="{x:Type DataGrid}" BasedOn="{StaticResource {x:Type DataGrid}}">
    <Setter Property="ColumnHeaderStyle" Value="{StaticResource DataGridColumnHeaderTallStyle}"/>
    <Setter Property="RowStyle" Value="{StaticResource DataGridRowTallStyle}"/>
</Style>

<!--No DataGridColumnHeaderTallStyle applied-->
<DataGrid Style="{StaticResource DataGridTallStyle}"/>

<!--Works as expected-->
<DataGrid Style="{StaticResource DataGridTallStyle}"
          ColumnHeaderStyle="{StaticResource DataGridColumnHeaderTallStyle}"/>

WpfDataGridColumnHeaderRepo.zip

[miloush]

https://learn.microsoft.com/en-us/dotnet/desktop/wpf/properties/dependency-property-value-precedence states that implicit style has precedence over style setter value. Local value has precedence over implicit style. That seems to describe what is happening, or did I get something wrong?

[brinko99]

@miloush Thanks for the reply. Mapping that precedence list into real-world effects has always been a challenge for me.

In this specific instance, you'll see that an implicit style for DataGridRow is also being defined with a MinHeight. However, the MinHeight as applied by DataGridRowTallStyle is being used.

That is, using <DataGrid Style="{StaticResource DataGridTallStyle}"/> does indeed cause DataGridRowTallStyle to be applied, but not DataGridColumnHeaderTallStyle

Why might this work for the row but not the column header?

[miloush]

Right I see the confusion here. Row is a bit special in that it is the item of the items control (the ItemsControl.ItemContainerStyle is coerced to RowStyle). The item container generator therefore effectively applies the RowStyle explicitly in code:

https://referencesource.microsoft.com/#PresentationFramework/src/Framework/System/Windows/Controls/ItemsControl.cs,3450

which makes it a local value, regardless of how the row style was originally set. That does not happen to the header style

[brinko99]

Yes, I see the special case with the RowStyle.

I still find myself a bit confused over the precedence acting on the header. I'd assumed that the issue was the fact that the implicit header style defined the MinHeight value which took precedence over that in my named style.

However, even after removing the setter, the effect remains.

<Style x:Key="{x:Type DataGridColumnHeader}" TargetType="{x:Type DataGridColumnHeader}">
    <!--<Setter Property="MinHeight" Value="28" />-->
</Style>

Is it really that attempting to set the ColumnHeaderStyle of DataGrid itself which is losing out in the precedence hierarchy? If so, it seems that setting a style from within a style is a no-no if there is an implicit style for a given type. True?

<Setter Property="ColumnHeaderStyle" Value="{StaticResource DataGridColumnHeaderTallStyle}"/>

Thanks again