Read original post by Jeremy Likness at C#er : IMage
Developers who are writing Windows Store apps using C# and XAML might find some of the support for Model-View-ViewModel (MVVM) lacking. Both WPF and Silverlight provided specific interfaces that enabled you to store validation context about fields on a context and even supported asynchronous validation. Although there are some existing frameworks that provide this support, such as Prism for the Windows Runtime, it is also fairly simple and straightforward to roll your own.