ILookup< TKey，TElement>在TElement中(被声明)是协变的? [英] Shouldn't ILookup<TKey, TElement> be (declared) covariant in TElement?

问题描述

定义System.Linq.ILookUp读取

The definition System.Linq.ILookUp reads

interface ILookup<TKey, TElement> : IEnumerable<IGrouping<TKey, TElement>>, IEnumerable {
    int Count {
        get;
    }

    IEnumerable<TElement> this[TKey key] {
        get;
    }

    bool Contains(TKey key);
}

由于IEnumerable在IGrouping< TKey，TElement> ;、 IGrouping< TKey，TElement>中是协变的.在TElement中是协变的，并且接口仅将TElement作为返回类型公开，我假设ILookup在TElement中也是协变的.确实，定义

Since IEnumerable is covariant in IGrouping<TKey, TElement>, IGrouping<TKey, TElement> is covariant in TElement and the interface only exposes TElement as a return type, I would assume that ILookup is also covariant in TElement. Indeed, the definition

interface IMyLookup<TKey, out TElement> : IEnumerable<IGrouping<TKey, TElement>>, IEnumerable {
    int Count {
        get;
    }

    IEnumerable<TElement> this[TKey key] {
        get;
    }

    bool Contains(TKey key);
}

编译没有问题.

那么，原始定义中缺少 out 关键字的原因可能是什么?可以添加Linq的将来版本吗?

So, what might be the reason why the out keyword is missing in the original definition? Might it be added future versions of Linq?

推荐答案

跟踪MSDN文档，

Tracking the MSDN documentation, Covariance and Contravariance in Generics have been introduced in .NET Framework 4 Prior to that, there was IEnumerable<T> since the .NET Framework 2.0 up to .NET Framework 3.5. Then in .NET Framework 4.0 we can see IEnumerable<out T> with type parameter T as covariance.

IGrouping<TKey, TElement>和ILookup<TKey, TElement>已经存在.在.NET Framework 4.0中，前者已更新为IGrouping<out TKey, out TElement>，但省略了后者，但未指定原因.

IGrouping<TKey, TElement> and ILookup<TKey, TElement> have existed since .NET Framework 3.5. In .NET Framework 4.0 the former has been updated to IGrouping<out TKey, out TElement> but the latter has been omitted without specifying the reason.

TKey不能是协变的，因为Contains(TKey)和this[TKey]的实现可以防止这种情况.

TKey can't be covariant since implementations of Contains(TKey) and this[TKey] prevent that.

关于TElement的问题尚不清楚.我不相信设计师会错过它.也许原因在于未来的计划.或者他们想防止出现以下情况，但我不知道为什么:

With regard to TElement the issue is not clear. I don't believe that designers just missed it. Perhaps cause lies in the plans for the future. Or they wanted to prevent something like the below, but I don't know why:

string[] strings = new[] {"a", "a", "b", "b", "b", "c"};
ILookup<string, string> lookup = strings.ToLookup(s => s); // Valid.
ILookup<string, object> lookup = strings.ToLookup(s => s); // Now invalid, but would correct if TElement was covariant (out TElement).

也有其他作者关注该问题:

There are also other authors, that pay attention to that issue:

ToLookup :

值得注意的一点是，尽管IGrouping在TKey和TElement中是协变的，但ILookup在两个类型参数中都是不变的.虽然TKey必须是不变的，但TElement是协变的是合理的

One slightly odd point to note is that while IGrouping is covariant in TKey and TElement, ILookup is invariant in both of its type parameters. While TKey has to be invariant, it would be reasonable for TElement to be covariant

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