是否可以覆盖属性并在VB.NET中返回派生类型？

Question

考虑以下表示订购系统的类:

Public Class OrderBase
    Public MustOverride Property OrderItem as OrderItemBase
End Class

Public Class OrderItemBase
End Class

现在,假设我们想要将这些类扩展为更具体的订单类集,保持OrderBase的聚合性质:

Public Class WebOrder
    Inherits OrderBase        

    Public Overrides Property OrderItem as WebOrderItem 
    End Property
End Class

Public Class WebOrderItem
    Inherits OrderItemBase
End Class

WebOrder类中的Overriden属性将导致错误,指出返回类型与OrderBase中定义的类型不同...但是,返回类型是OrderBase中定义的类型的子类.为什么VB不允许这样？

Answer 1

你不能这样做 - 它正在改变基础上定义的签名.要做你想做的事,你需要使用泛型:

Public Class OrderBase(Of T As IOrderItem)
    Public ReadOnly Property OrderItems As IList(Of T)
End Class

我的Visual Basic生锈了所以希望这是准确的......

Answer 2

您不能在重写类时更改其签名。但是，您可以返回派生类型：

Public Overrides Property OrderItem() as OrderItemBase
    Get
        Return New WebOrderItem()
    End Get
End Property

Public Sub Whatever()
    Dim item As WebOrderItem = DirectCast(OrderItem, WebOrderItem)
End Sub

或者，如果要更严格地强制类型，请使用具有泛型类型约束的泛型，如下所示：

Public MustInherit Class OrderBase(Of T As OrderItemBase)
    Public MustOverride ReadOnly Property OrderItem() As T
End Class

Public Class OrderItemBase
End Class

Public Class WebOrder(Of T As WebOrderItem)
    Inherits OrderBase(Of T)

    Public Overrides ReadOnly Property OrderItem() As T
        Get
            Return New WebOrderItem()
        End Get
    End Property
End Class

Public Class WebOrderItem
    Inherits OrderItemBase
End Class

如果您不希望WebOrder也成为通用类，请执行以下操作：

Public Class WebOrder
    Inherits OrderBase(Of WebOrderItem)

    Public Overrides ReadOnly Property OrderItem() As WebOrderItem
        Get
            Return New WebOrderItem()
        End Get
    End Property
End Class