Imagine, we have a wpf class library with a window1.xaml and a resource dictionary res.xaml defined in it. I know how to use styles that defined in the res.xaml for the controls that defined into the window: <Window x:Class="...Window1"> <Window.Resources> <ResourceDictionary> <ResourceDictionary.MergedDictionaries> <ResourceDictionary Source="res.xaml"/> </ResourceDictionary.MergedDictionaries> </ResourceDictionary> <\Window.Resources> </Window> So we can use the dictionary's styles for all elements into the window (except the window element... I don't know how to set the style from the res.xaml for the window :( ). I saw the article where describes how to create and use attached property to add resource dictionaries to a FrameworkElement.Resources.MergedDictionaries list. It's good! We can do the same as we done in the example above but we can use the window style now. It looks like this: <Window x:Class="...Window1" xmlns: resources="..." resources:SharedResources.MergedDictionaries="res.xaml"> </Window> That's good but VS2008 cannot recognize resources from res.xaml in design time. So we have a sad situation: all styles from res.xaml are available in run-time but in the design-time VS cannot display the window (it can't find the mentioned styles). Does anybody know how to fix this situation?