Question

Can someone explain not what MustOverride does, but why use it? Is it to expose the function?

I have two classes, the first (RoomFactory);

Public MustInherit Class RoomFactory : Inherits baseFactory
Private _roomid As Integer = 0
Private _roomname as String = ""

Public Sub New()

End Sub

Public Sub New(ByVal roomid As Integer, ByVal roomname As String)
    Me.RoomId = roomid
    Me.RoomName = roomname
End Sub

Public MustOverride Function CreateRoom(ByVal roomdetails As RoomFactory) As Integer
Public MustOverride Function IsRoomAvailable(ByVal roomdetails as RoomFactory) As Boolean
// .. properties removed for brevity .. //

Second class (Room)

Public Class Room : Inherits RoomFactory
    Public Function CreateRoom(ByVal roomdetails As RoomFactory) As Integer
        Return 0
    End Function
    Public Function IsRoomAvailable(ByVal roomdetails As RoomFactory) As Boolean
        Return False
    End Function
End Class

Firstly, I think this is right, but would like any advice to the otherwise – performance etc. But I guess the primary question is – why use the MustOverride?

Please excuse my ignorance here.

Answer 1

It’s so that you can provide common functionality in a base class, but force derived classes to implement specific bits of functionality themselves.

In your factory situation I would suggest using an interface rather than an abstract class, but in other cases it makes sense. System.Text.Encoding is a good example of an abstract class, as is System.IO.Stream.