The normal way to apply a platform specific custom renderer is like this:
[assembly: ExportRenderer(typeof(PlatformSpecificContentPage), typeof(UWPContentPageRenderer))]
This blanket affects all classes of type PlatformSpecificContentPage in the system. But, I want to be able to apply a custom renderer to say "Entry" controls through Xaml. e.g. in some cases, I want to replace an TextBox in UWP with a custom control that has syntax highlighting, but not in all cases. The problem with the ExportRendererAttribute is that I have to inherit from Entry in order to apply the custom renderer. This is a pretty silly design flaw of the system.