性質調節

Question

我有結構化這樣的類：

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 

是否有修改，我可以在d類屬性X使用，這將導致X從得自d的實例和返回爲B評估爲C的D的實例？

即

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

在這兩種情況下，我要X到不能爲Nothing

我知道我可以修改d如下：

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 

是否有一個清晰的解決方案？

Answer 1

你應該使用Shadows避免新的代碼，但如果你真的需要這個結構，我明白你只用添加MyBase._X=_X到D的預期結果New（選項1），它也可以做通過D調用C中的New，然後進行相反的賦值_X = DirectCast(MyBase._X, B)（選項2）。

當然另一種選擇，這取決於你的實際意圖，是砸在Private _X As B和D投只是在_X到B在你的陰影X。 （選項3）

這是我的代碼與選項1和2註釋和當前執行選項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