Question

I have classes structured like this:

Public MustInherit Class A
  ' several properties
End Class

Public Class B
  Inherits A
  ' several properties
End Class

Public MustInherit Class C
  Protected _X As A
  Public ReadOnly Property X As A
    Get
      Return _X
    End Get
  End Property
End Class

Public Class D
  Inherits C
  Private _X As B
  Public ReadOnly Property X As B
    Get
      Return _X
    End Get
  End Property
  Sub New
    _X = New B
  End Sub
End Class

Is there a modifier I can use on property X in class D which will cause X to be returned as B from an instance of D and A from an instance of D that is evaluated as C?

i.e.

Dim d As New D
Response.Write((d.X Is Nothing) & "<br>")
Dim c As C = d
Response.Write(c.X Is Nothing)

In both cases I want X to not be Nothing

I realize I could modify D as follows:

Public Class D
  Inherits C
  Private __X As B
  Public ReadOnly Property X As B
    Get
      Return __X
    End Get
  End Property
  Sub New
    __X = New B
    _X = __X
  End Sub
End Class

Is there a cleaner solution?

Answer 1

You should avoid new code using Shadows, but if you really need this construct, I get the results you’re expecting with just the addition of MyBase._X=_X to D‘s New (Option 1) which could also be done by having a New in C that D calls, then does the opposite assignment _X = DirectCast(MyBase._X, B) (Option 2).

Of course another option, depending upon your actual intention, is to drop the Private _X As B in D and just cast the _X to B in your shadowed X. (Option 3)

Here is my code with Options 1 and 2 commented out and currently performing Option 3.

Public MustInherit Class C
    Protected _X As A
    ''Option 2:
    'Protected Sub New(X As A)
    '    _X = X
    'End Sub
    Public Overridable ReadOnly Property X As A
        Get
            Return _X
        End Get
    End Property
End Class

Public Class D
    Inherits C
    'Commented out for option 3:
    'Private Shadows _X As B
    Public Shadows ReadOnly Property X As B
        Get
            'Commented out for option 3:
            'Return _X
            Return DirectCast(_X, B)
        End Get
    End Property
    Sub New()
        ''Option 1,3: (Comment out for option 2)
        _X = New B
        ''Option 1:
        'MyBase._X = _X
        ''Option 2:
        'MyBase.New(New B)
        '_X = DirectCast(MyBase._X, B)
    End Sub
End Class