Actually, the View control can't be used as a base class for the return type (I'm not sure why), but we have to use another View-derived control like a Grid or any layout. Also, the above use of the <OnIdiom> markup is invalid. This markup does not use the <On> tag, nor Idiom attribute like what <OnPlatform> does. It should be like:

<OnIdiom x:TypeArguments="Grid"> <OnIdiom.Desktop> <views:DesktopView /> </OnIdiom.Desktop> <OnIdiom.Phone> <views:MobileView /> </OnIdiom.Phone> </OnIdiom> May I ask Microsoft, where is constancy? You can also use these markups in a short-handed way:

<Grid BackgroundColor="{OnIdiom Desktop=AliceBlue, Phone=Aqua}"/> Thanks for the great work! Hashnode first experience to create an account is a matter of time-waste though.