WPF Style for base window not applied in App.xaml, but is in Themes/Generic.xaml

Question

I am in the process of creating a base window class for most of my windows to derive from. Obviously the best solution for this was a separate class, and a style that applies to it.

The issue is that the <Style ../> I have is not being applied when it is in App.Resources. That is, if it's defined in an external ResourceDictionary, and merged into App.xaml's resources, or a local dictionary and merged, or placed inline into App.Resources. The <Style ../> is, however, applied when it is placed into Themes/Generic.xaml.

The problem can be demonstrated without doing anything special at all in the base window, apart from overriding the DefaultStyleKeyProperty.

Below is ThemeWindow:

public class ThemeWindow : Window
{
    static ThemeWindow()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(ThemeWindow), new FrameworkPropertyMetadata(typeof(ThemeWindow)));
    }
}

Here is the very simple <Style ../> I am trying to apply (it makes the Window background red, nothing more):

<Style TargetType="{x:Type testing:ThemeWindow}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type testing:ThemeWindow}">
                <Grid>
                    <Grid.Background>
                        <SolidColorBrush Color="Red"/>
                    </Grid.Background>
                    <AdornerDecorator>
                        <ContentPresenter />
                    </AdornerDecorator>
                </Grid>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

The MainWindow that uses ThemeWindow, is simply the following XAML:

<testing:ThemeWindow x:Class="Testing.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:testing="clr-namespace:Testing"
        Title="MainWindow" Height="350" Width="525">
    <Grid>
        <Button Content="Button" HorizontalAlignment="Left" Margin="125,83,0,0" VerticalAlignment="Top" Width="75"/>
    </Grid>
</testing:ThemeWindow>

Now, as stated, if you place that Style in its own ResourceDictionary, and include it like this:

<App.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Themes/ThemeWindow.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</App.Resources>

.. it does not work. If you inline the style straight into App.Resources, it does not work.

The only situation I can find it working is to call the ResourceDictionary xaml Generic.xaml, and place it into the Themes/ directory of the application.

I am wondering exactly why this is happening.

My only theory is that when WPF sees a control type, it will head over to Themes, and scan all ResourceDictionarys for the type, then fall back to Generic.xaml and load it. This doesn't explain why it would not load if the <Style /> is available in a merged ResourceDictionary though. Note that it does work if the MergedDictionary is placed into Generic.xaml, for obvious reasons.

I'm perfectly fine with having to merge the ResourceDictionary into Generic.xaml if that's what I have to do. I just want to get down at the technical details as to why it needs to be like this.

Screenshots of this not working / working:

Answer 1

I have a simple workaround that would allow you to set your Style in you app.xaml.

Define your style in app.xaml like this :

<Style x:Key="{x:Type testing:ThemeWindow}" TargetType="{x:Type testing:ThemeWindow}">

And change your ThemWindow to this :

public class ThemeWindow : Window
{
    static ThemeWindow()
    {
        StyleProperty.OverrideMetadata(typeof(ThemeWindow), new FrameworkPropertyMetadata(GetDefautlStyle()));
    }

    private static Style GetDefautlStyle()
    {
        if (defaultStyle == null)
        {
            defaultStyle = Application.Current.FindResource(typeof(ThemeWindow)) as Style;
        }
        return defaultStyle;
    }

    private static Style defaultStyle = null;
}

It does not really solve the question, but that would allow you to achieve what you need !

EDIT : Looking at DefaultStyleKey reference, it's clearly stated that it's used for theme style lookup. That explains why it won't find it in app.xaml or any other dictionary. It will only search in Theme dictionaries. So you either have to define your style in a Theme Dictionary, or to use the Style property directly as in the above example.