The name "ContentPage" does not exist in the namespace "http://xamarin.com/schemas/2014/forms".

Hi
I am New to Xamarin. I have a Code For IOS and Android App using Xamarin Forms. Now i want to Develope for the Windows Phone using Shared Code. I am Getting an Error in the Forms used in the Shared Code as The name "ContentPage" does not exist in the namespace "http://xamarin.com/schemas/2014/forms".