vb.net - 泛型、接口(interface)和强制转换问题

Question

我最近为我实现的一些自定义用户控件添加了一个界面。界面非常基本。它有一种支持链接的方法:

Public Interface IMyInterface(Of T As WebControl)
    Function DoSomething() As T
End Interface

实现也很基本:

Public Class MyCustomControl
    Inherits CompositeControl
    Implements IMyInterface(Of MyCustomControl)

Public Function DoSomething() As MyCustomControl _
    Implements IMyInterface(Of MyCustomControl).DoSomething
    ' do stuff

    Return Me
End Class

到目前为止一切正常。当我尝试遍历所有实现 IMyInterface 的控件集合时，就会出现问题。界面，像这样:

Dim myList = New List(Of IMyInterface(Of WebControl))

myList.Add(someCustomControl)

myList.ForEach(Sub(i) i.DoSomething())

someCustomControl是 MyCustomControl实现IMyInterface(Of MyCustomControl)而不是 IMyInterface(Of WebControl) .

我在第二行收到此错误(我尝试添加 someCustomControl ):

Option Strict On disallows implicit conversions from 'MyCustomControl' to 'IMyInterface(Of WebControl)'.

有没有办法解决这个错误？我已经接近让它工作了，但我对泛型的了解还不够，无法超越这一点。

Answer 1

协方差是 VS 2010 中引入的一种语言功能，可以解决您的问题。您需要定义您的泛型，以便类型 T有Out前面的关键字:

Public Interface IMyInterface(Of Out T As WebControl)
    Function DoSomething() As T
End Interface

When you use the Out keyword, you are using covariance. It allows generics of a more derived type to be used in place of a generic with the base type. So in your case it will allow a IMyInterface(Of MyCustomControl)) object in places where the code would normally expect IMyInterface(Of WebControl)), such as your for loop.

Note that covariance has a restriction. The covariant type T can only be used as a function return value, and not as a parameter into a function (or sub). For example, if the DoSomething signature in IMyInterface looked like this the compiler would complain:

' Here the type T is used as an input param - compiler error
Sub DoSomething(ByVal sampleArg As T)

鉴于您的链接场景，我认为上述限制不是问题。

MSDN 上的更多信息:

Covariance and Contravariance

Creating Variant Generic Interfaces