XAML designer with UserControls in DLL doesn't resolve resource - wpf

Question

I have a solution in Visual Studio 2019, made of some projects built as DLL using WPF .NET 5.
Now I need to create some UserControls inside one of this projects, that I will load in a future WPF .NET 5 application, but the problem is that the XAML designer is not able to preview the UserControl with StaticResource that are in an external ResourceDictionary.
I simplify the code:
<UserControl.Resources>
<Style x:Key="sButton" TargetType="Button">
<Setter Property="BorderBrush" Value="{StaticResource color_UC_BorderBrush}"/>
</Style>
</UserControl.Resources>
Having only DLL I don't have an Application with shared ResourceDictionary and color_UC_BorderBrush could not be resolved for preview.
In the following way instead the XAML designer works properly, but I don't want that every single UserControl loads his own ResourceDictionary everytime:
<UserControl.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="pack://application:,,,/MyLib;component/ResourceDictionaries/myRD.xaml"/>
</ResourceDictionary.MergedDictionaries>
<Style x:Key="sButton" TargetType="Button">
<Setter Property="BorderBrush" Value="{StaticResource color_UC_BorderBrush}"/>
</Style>
</ResourceDictionary>
</UserControl.Resources>
In my future WPF .NET 5 application I will use shared ResourceDictionary but for now how can I manage to let the XAML designer works properly inside a DLL or how can I make the MergedDictionary defined in the XAML ignorable?
Maybe there are other ways I don't have in mind. Thank you.

Answer 1

You can't omit merging the resource into a ResourceDictionary within the scope of the control that depends on it.
But instead of merging the same resource into each control's ResourceDictionary, you can choose a ResourceDictionary of a common parent. The XAML resource lookup strategy will traverse the logical tree to search for the required resource, starting at the local ResourceDictionary of the control and ending at the root, before checking App.xaml and Generic.xaml.
In case of the UserControl, you would have to use the DynamicResource markup extension. This is, because the content will only be part of of the logical tree during runtime. In design time, the root from the user control's content point of view is the containing UserControl.
You should know that DynamicResource has a performance penalty. So you typically would try to avoid it.
Also defining the resource in each control's ResourceDictionary will introduce a performance penalty.
The best solution is to define the resource in the application's root control like Window.Resources or even better in the ResourceDictionary inside App.xaml.
Using App.xaml to host the merged ResourceDictionary would also allow to use the cheaper StaticResource from inside the UserControl to lookup the merged resources.
Please read Static and dynamic resources to learn more about the lookup behavior of the XAML engine.
The following example chooses the MainWindow as the host for the merged resources. It is the root of all it's contained elements and therefore reachable from any location in its logical tree.
MainWindow.xaml
Make the resource available for any element that is a child of MainWindow:
<Window>
<Window.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<!-- The resource dictionary that contains the shared 'color_UC_BorderBrush' resource -->
<ResourceDictionary Source="pack://application:,,,/MyLib;component/ResourceDictionaries/myRD.xaml" />
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Window.Resources>
<MyUserControl />
</Window>
MyUserControl.xaml
<UserControl>
<UserControl.Resources>
<Style x:Key="sButton" TargetType="Button">
<Setter Property="BorderBrush"
Value="{DynamicResource color_UC_BorderBrush}" />
</Style>
</UserControl.Resources>
<Button Style="{StaticResource sButton}" />
</UserControl>