Xamarin form reflection error with windows

reading in the official guide I tryed to use the reflection on a common file in a Xamarin form shared project.

My problems are with those rows:

this.GetType().GetProperties()

and

property.PropertyType.GetGenericTypeDefinition()

It always says that it is working on xamarin.droid - xamarin.ios, but never on xamarin.windows - xamarin.winPhone.

I took some of this code from a windows phone project (no xamarin) and it was perfectly working, why now I have this problem?

thanks

PS: I'm using xamarin with Visual Studio 2015