Couldn't find the exact scenario I'm trying to produce in other questions.
What I'm trying to do is:
BaseApplication
* App.xaml
* App.xaml.cs
SpecificApplication
* App.xaml : BaseApplication.App.xaml
* App.xaml.cs : BaseApplication.App.xaml.cs
But I get this:
'BaseApplication.App' cannot be the root of a XAML file because it was defined using XAML.
Does this mean BaseApplication's App class cannot have XAML?
