Is it possible to override a property and return a derived type in VB.NET?

Question

Consider the following classes representing an Ordering system:

Public Class OrderBase
    Public MustOverride Property OrderItem as OrderItemBase
End Class
         


        

Answer 1

One approach is to have a protected overridable method, and then have a public non-overridable method which calls the overridable one. Any time the return value for the function in the derived class should change, have a notoverridable override of the overridable method call a new overridable method which returns the more refined type, and also shadow the earlier version of the public function with one that uses the new override. If vb.net allowed one class to both override and shadow the same member, things would be much cleaner, but there's no way to do that.

Public Class CarFactory
  Protected Overridable Function DerivedMakeCar() as Car
    ' make a car
  End Function

  Public Function MakeCar() as Car
    Return DerivedMakeCar()
  End Function

End Class

Public Class FordFactory
  Inherits CarFactory

  Protected Overrides Function DerivedMakeCar() As Car
    Return DerivedMakeFord()
  End Function

  Protected Overridable Function DerivedMakeFord() As Ford
    ' Make a Ford
  End Function

  Public Shadows Function MakeCar() As Ford
    Return DerivedMakeFord()
  End Function

End Class

A simpler alternative in some cases may be to have a public overridable MakeCar() function which always returns an object of type Car, but have a FordFactory also include a MakeFord() function which returns a Ford.

The overridden MakeCar() function would be NotOverridable and would simply call MakeFord. In some ways, the latter approach can be cleaner, but if there's a common naming convention (e.g. factories have a MakeProduct method which returns the most derived type) it may be useful to use Shadows.