I am working on a travel site application that illustrates inheritance and
polymorphism.  The abstract base class is Reservable.  The three derived
classes are Hotel, Car, and Flight.

I have the following method, which acts as a controller class between the
form and the above classes.  The method's purpose is to accept a Hotel, Car,
or Flight object and add it to theReservations ArrayList:
Public Class ReservableController
   Private theReservations As System.Collections.ArrayList

   Public Sub addReservation(ByRef aReservation As Reservable)
       theReservations.Add(aReservation)
   End Sub

...
End Class

I receive an object reference error when it attempts to run the Add method.
aReservation is not Nothing when it hits the method.  Any ideas?

Mark Serva
The University of Delaware