如何避免向下转换为接口类？

Question

我正在使用Excel VBA(Excel 2010),并且在尝试使用继承时遇到了问题.基本上,我有一个接口MyInterface和一个实现类MyImplementation.在VBA代码中,当我引用一个Dim类型时MyInterface,我只能访问在该接口上定义的成员 - 这是预期的.当我引用一个Dim类型时MyImplementation,我无法访问它实现的接口上定义的成员 - 不是预期的.

为什么我不能直接在实现类上调用interface属性？

MyInterface的

Option Explicit

Public Property Get Text() As String
End Property

MyImplementation

Option Explicit
Implements MyInterface

'The implementation of the interface method'
Private Property Get MyInterface_Text() As String
  MyInterface_Text = "Some Text"
End Property

Public Property Get MoreText() As String
  MoreText = "Yes, some more text!"
End Property

MainModule - 用法示例

Function Stuff()
  Dim impl As New MyImplementation
  Dim myInt As MyInterface: Set myInt = impl
  'The following line is fine - displays "Yes, some more text!"
  MsgBox impl.MoreText
  'This is also fine - displays "Some text"
  MsgBox DownCast(impl).Text
  'This is also fine - displays "Some text"
  MsgBox myInt.Text
  'This is *not* fine - why??
  MsgBox impl.Text
End Function

Function DownCast(ByRef interface As MyInterface) As MyInterface
  Set DownCast = interface
End Function

主要问题是如何避免向下投射？

注意 - 上面的例子是有意设计的.我意识到直接引用实现类通常是不好的做法.

Answer 1

当我引用MyImplementation类型的Dim时,我无法访问它实现的接口上定义的成员 - 不是预期的.

解决方案是改变您的期望.这就是VBA中的工作方式:VBA类实现COM接口(例如IUnknown)而不公开它们.

如果要从类中公开接口的成员,则必须明确地这样做:

Option Explicit
Implements MyInterface

'The implementation of the interface method'
Private Property Get MyInterface_Text() As String
    MyInterface_Text = "Some Text"
End Property

Public Property Get MoreText() As String
    MoreText = "Yes, some more text!"
End Property

Public Property Get Text() As String
    Text = MyInterface_Text 
End Property