C# does not support multiple inheritance, so you cannot do the following line:
public class My_App_ItemViewer : abstractItemViewer, System.Web.UI.Page, System.Web.UI.ICallbackEventHandler
Just to clarify, the way you have written the above, the C# compiler thinks that abstractItemViewer is a class that you are trying to inherit from. The compiler then sees the "System.Web.UI.Page" part and is looking for an interface named that, it does not find it, because System.Web.UI.Page is a class and not an interface; thus error.
You can, however, implement multiple interfaces, so you could do the following:
public class My_App_ItemViewer : System.Web.UI.Page, System.Web.UI.ICallbackEventHandler, IAbstractItemViewer