Question

I am building some WPF pages in a win forms application. I wish to use WPF Themes for my application. Not having an App.xaml (because the project is a win forms project with a WPF ElementHost to show the WPF forms) I added my theme resource dictionary in my form like this:

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Themes/ExpressionDark.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</UserControl.Resources>

This works for all controls on this form plus some of the other WPF user controls, but there are some user controls where the theme is not functioning. I have tracked down that the affected controls that define their own resources like this:

    <Grid.Resources>
        <Style TargetType="ComboBox">
            <Setter Property="Margin" Value="0 2 5 2" />
        </Style>
    </Grid.Resources>

It doesn’t seem to matter where the resources are located or what is in the resource. Anything in the resource that is not keyed blocks the theme for everything that style targets. I even tried to out-smart it by finding the resource in the dictionary:

<Style TargetType="{x:Type ComboBox}">
    <Setter Property="Foreground" Value="{DynamicResource TextBrush}" />
    <Setter Property="SnapsToDevicePixels" Value="true" />
    <Setter Property="Template" Value="{DynamicResource ComboBoxTemplate}" />
</Style>

And modifying my style to look like this:

    <Grid.Resources>
        <Style TargetType="ComboBox">
            <Setter Property="Foreground" Value="{DynamicResource TextBrush}" />
            <Setter Property="SnapsToDevicePixels" Value="true" />
            <Setter Property="Template" Value="{DynamicResource ComboBoxTemplate}" />
            <Setter Property="Margin" Value="0 2 5 2" />
        </Style>
    </Grid.Resources>

But that caused my combo boxes to disappear completely!

Is there any way to add resources that blanket target controls and not throw off the theme?

Answer 1

You just need to set the BasedOn property of the Style to the old implicit Style:

<Grid.Resources>
    <Style x:Key="ComboBoxStyle" 
           TargetType="{x:Type ComboBox}" 
           BasedOn="{StaticResource {x:Type ComboBox}}">
        <Setter Property="Margin" Value="0 2 5 2" />
    </Style>

</Grid.Resources>

edit

So, if you don’t want to use the resource key on all of your ComboBox’s, you have to take it one step further. I don’t know why WPF / WinForms interop handles implicit styles so poorly, but even if you add an implicit style to the resources here, and set BasedOn="{StaticResource ComboBoxStyle}", you will still lose your template.

However, if you move the resources to the UserControl, you can then do the implicit style properly:

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Themes/ExpressionDark.xaml" />
        </ResourceDictionary.MergedDictionaries>

        <Style x:Key="ComboBoxStyle" 
               TargetType="{x:Type ComboBox}"
               BasedOn="{StaticResource {x:Type ComboBox}}">
            <Setter Property="Margin" Value="0 2 5 2" />
        </Style>
    </ResourceDictionary>
</UserControl.Resources>

<Grid>
    <Grid.Resources>
        <Style TargetType="{x:Type ComboBox}" 
                   BasedOn="{StaticResource ComboBoxStyle}" />
    </Grid.Resources>