有人能解释一下吗?

我有两个班,第一个(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 .. //

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

首先,我认为这是正确的,但我想对其他方面提出建议-性能等。但我想主要问题是-为什么要使用MustOverride?