I've found that the base class "UserControl" is declared in both the codebehind file and in the generated partial class. This behavior does not permit to inherit user controls from a custom base class deriving from usercontrol itself. Here is an example of the code I would like to write:
public class ProductPage : SceneUserControl
public class SceneUserControl : UserControl
Other platforms like ASP.NET grant the capability of inheriting Pages or UserControl from a common base class implmenting common behaviors.