为什么在实现接口时不能使用兼容的具体类型 [英] Why can't I use a compatible concrete type when implementing an interface

问题描述

我希望能够做这样的事情:

I would like to be able to do something like this :

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Test
{
    public interface IFoo
    {
        IEnumerable<int> integers { get; set; }
    }

    public class Bar : IFoo
    {
        public List<int> integers { get; set; }
    }
}

为什么编译器会抱怨..?

Why does the compiler complains..?

Error   2   'Test.Bar' does not implement interface member 'Test.IFoo.integers'. 'Test.Bar.integers' cannot implement 'Test.IFoo.integers' because it does not have the matching return type of 'System.Collections.Generic.IEnumerable<int>'.

我知道接口说 IEnumerable 并且该类使用列表，但是列表 is 是 IEnumerable.....

I understand that the interface says IEnumerable and the class uses a List, but a List is an IEnumerable.....

我能做什么?我不想在类中指定 IEnumerable，我想使用实现 IEnumerable 的具体类型，比如 List...

what can I do? I do not want to specify IEnumerable in the class, I want to use a concrete type that implements IEnumerable, like List...

推荐答案

这是一个类型协方差/逆变问题(参见 http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#C.23).

This is a Type Covariance/Contravariance issue (see http://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science)#C.23 ).

有一个解决方法:使用显式接口，如下所示:

There's a workaround: use explicit interfaces, like so:

public class Bar : IFoo {

    private IList<int> _integers;

    IEnumerable<int> IFoo.integers {
        get { return _integers };
        set { _integers = value as IList<int>; }
    }

    public IList<int> integers {
        get { return _integers; }
        set { _integers = vale; }
    }
}

请注意，integers 应为 TitleCased 以符合 .NET 的准则.

Note that integers should be TitleCased to conform to .NET's guidelines.

希望你能在上面的代码中看到问题:IList 与 IEnumerable 兼容，仅适用于访问器，不适用于设置.如果有人调用 IFoo.integers = new Qux()(其中 Qux : IEnumerable 但 not Qux : IList).

Hopefully you can see the problem in the code above: IList<int> is compatible with IEnumerable<int> only for the accessor, but not for setting. What happens if someone calls IFoo.integers = new Qux<int>() (where Qux : IEnumerable<int> but not Qux : IList<int>).

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