为什么接口IOrderedEnumerable< T> T不是协变的吗？

问题描述

我正在查看IOrderedEnumerable的声明，我很惊讶它的TElement类型参数不是协变的。

I was Looking at the declaration of IOrderedEnumerable an I was suprised that it isn't covariant in it's TElement type parameter .

public interface IOrderedEnumerable<TElement> : IEnumerable<TElement>, IEnumerable
{
    IOrderedEnumerable<TElement> CreateOrderedEnumerable<TKey>(Func<TElement, TKey> keySelector, IComparer<TKey> comparer, bool descending);
}

不进行协变的原因是什么？

What's the reason for which it was not made covariant ?

推荐答案

这是一个疏忽，并且已在.NET Core中修复。这是关于此的（已关闭），这里是对其进行了修复。

It's an oversight and that was fixed in .NET Core. Here is (closed) issue about that and here is pull request which fixes it.

它没有完全固定我认为.NET版本是一个重大变化。例如（想法取自此，它涉及另一个重大更改，但也适用于此）：

It's not getting fixed in full .NET version I think because that's a breaking change. For example (idea is taken from this answer which is about another breaking change, but applies here too):

public class Base
{
    public void DoSomething(IOrderedEnumerable<string> strings)
    {
        Console.WriteLine("Base");
    }
}

public class Derived : Base
{
    public void DoSomething(IOrderedEnumerable<object> objects)
    {
        Console.WriteLine("Derived");
    }
}

然后致电

Derived d = new Derived();
d.DoSomething(new List<string>().OrderBy(c => c));

如果 IOrderedEnumerable 不是协变的- Base 方法将被调用。现在假设我们将其更改为协变。下次编译此代码时，突然调用 Derived 方法。

If IOrderedEnumerable is not covariant - Base method would be called. Now suppose we change that to covariant. When we next time compile this code, suddenly Derived method is called.

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