I guess this is one of these places where using Blend is an advantage… since I do these operations mostly with the help of the tool, the XAML is generated and I don’t need to think about it too much.

There is a trick when you add objects (for example Styles or a ViewModelLocator) to App.xaml when it already has merged resource dictionaries. The syntax is a bit convoluted, so here we go:

<Application.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="Skins/Brushes.xaml"/>
        </ResourceDictionary.MergedDictionaries>

        <vm:ViewModelLocator x:Key="Locator"
                                d:IsDataSource="True" />
    </ResourceDictionary>
</Application.Resources>

So yeah, the trick is to put the ViewModelLocator (or any other object for that matter) within the ResourceDictionary tag…

HTH,

Laurent