Device.OnPlatform (T, T, T)' is obsolete: 'Use switch (RuntimePlatform) instead

I'm following xamarin-forms guide (https://developer.xamarin.com/guides/xamarin-forms/xaml/xaml-basics/xaml_markup_extensions/#The_xStatic_Markup_Extension) and trying out the AppConstants.cs example i got the following warning (see attached screenshot) :
"Warning CS0618: 'Device.OnPlatform (T, T, T)' is obsolete: 'Use switch (RuntimePlatform) instead.' ".
Can you suggest how modify AppConstants.cs instead of guide's version ?
many thanks