是否可以覆盖属性并在VB.NET中返回派生类型？ [英] Is it possible to override a property and return a derived type in VB.NET?

问题描述

考虑以下代表订购系统的类：

Consider the following classes representing an Ordering system:

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到更具体的订单类集，保持OrderBase的聚合性质：

Now, suppose we want to extend these classes to a more specific set of order classes, keeping the aggregate nature of OrderBase:

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属性将导致错误声明返回类型与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不允许这样做？

The Overriden property in the WebOrder class will cause an error stating that the return type is different from that defined in OrderBase... however, the return type is a subclass of the type defined in OrderBase. Why won't VB allow this?

推荐答案

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：

You can't do that - it's changing the signature defined on the base. To do what you are trying to do you need to use generics:

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

我的Visual Basic是生锈，所以希望这是准确的......

My Visual Basic is rusty so hopefully that is accurate...

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