title: ObservableCollection in the service layer of the WPF MVVM application
body: <p>Examples of WPF MVVM apps I've seen on the Internet consider VM a layer which interacts with a service layer which either uses "old" events from an external library, or interacts with web using HTTP or whatever. But what if I build all M, V, VM, service and other parts myself? How to properly build interaction between the service layer and the viewmodel layer? Can I just put <code>ObservableCollection&lt;OrderModel&gt;</code> into the service and return it as is from the viewmodel for the view, or is it considered a bad approach and there're better alternatives?</p>