ByRef :ByRef-在父类型中，但返回子类型为ByRef

Question

在这种情况下，如何将子类型返回给ByRef引用的变量？

Public Class Father
End Class

Public Class Son
   Inherits Father
End Class

' Should return a Son, not a Father
Public Sub Save(ByRef obj as Father)

Answer 1

但是，如果您愿意，您可以分配一个Son：

' Should return a Son, not a Father
Public Sub Save(ByRef obj As Father)
    obj = New Son
End Sub

现在，当您调用Save时，您拥有的是Son而不是Father

Dim father As New Father
Save(father)
If TypeOf father Is Son Then
    Console.Write("Yes, i'm a son") ' this is executed '
End If

您必须对其进行强制转换才能将其用作Son

If TypeOf father Is Son Then
    Dim son = DirectCast(father, Son)
End If