Hi,@HoWe Yu. Welcome to Microsoft Q&A.
You could try the following method to reference global resources in user controls
In App.xaml, make the following adjustments to <Application.Resources>
<Application.Resources>
<ResourceDictionary>
<ResourceDictionary.MergedDictionaries>
<ResourceDictionary Source="GlobalRes.xaml"></ResourceDictionary>
</ResourceDictionary.MergedDictionaries>
</ResourceDictionary>
</Application.Resources>
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