Question

I have a really contrived example to as how my business objects load their data. However I’d like to prevent the situation demonstrated below. I know there are shortcomings here, but it’s just how it is (so far)! How can I achieve this without restructuring to interfaces etc?

Public Class Car

  Public Property Passengers As Integer

  Public Shared Sub Fill(c As Car, r As SqlDataReader)
    Passengers = r.GetInt32(0)
  End Sub

End Class

Public Class UsedCar
  Inherits Car

  Public Property PreviousOwner As String

  Public Shared Sub FillUsed(l As Limo, r As SqlDataReader)
    Passengers = r.GetInt32(0)
    PreviousOwner = r.GetString(1)
  End Sub

End Class

Public Sub CreateVehicles()

  Dim c As New Car()
  Car.FillWithStuff(c, GetSqlReader(carId))

  Dim l As New UsedCar()
  UsedCar.FillUsed(l, GetSqlReader(usedCarId)) 'Great
  Car.Fill(l, GetSqlReader(usedCarId)) 'No! << Stop this at once!
  UsedCar.Fill(l, GetSqlReader(usedCarId)) '<<< Even more wrong!

End Sub

Answer 1

There is no way to require that except to throw an exception. For instance:

Public Shared Sub FillWithStuff(c As Car)
    If Not (TypeOf c Is Car) Then
        Throw New Exception("Must be a Car")
    End If
    Passengers = 4
End Sub

However, the simplified example you provide doesn’t adequately explain why, in your situation, a limo is not a valid car. If, in fact, it is not a valid car, then it should not inherit from car. It is likely that your code ought to be refactored so that car and limo are two unrelated classes that both share the same dependency business class for common functionality.