为什么通用的IList＆LT;＆GT;不继承非泛型的IList [英] Why generic IList<> does not inherit non-generic IList

问题描述

的IList＆LT; T＆GT; 不继承的IList ，其中的IEnumerable＆LT;出T＆GT; 继承的IEnumerable 。

如果退出修正是唯一的原因，那么为什么大多数的IList＆LT的执行情况; T＆GT; （如收藏＆LT; T＆GT; ，名单，其中，T＆GT; ）实现的IList 接口。

If out modifier are the only reason then why most of the implementation of IList<T> (e.g. Collection<T>, List<T>) implements IList interface.

因此​​，任何人能说好，如果这语句是适用于所有实施的IList＆LT的; T＆GT; 然后直接将其转换为的IList 在必要的时候。但问题是，虽然的IList＆LT; T＆GT; 不继承的IList ，因此不能保证每个的IList＆LT; T＆GT; 对象是的IList

So any one can say OK, if that statements is true for all implementation of IList<T> then directly cast it to IList when necessary. But problem is that though IList<T> does not inherit IList so it is not guaranteed that every IList<T> object are IList.

此外使用的IList＆LT;对象＆gt; 显然不是解决办法，因为没有退出修改仿制药不能分配到一个不太继承类;并创建目录的新实例是不是一个解决方案，在这里，因为有人可能希望的IList＆LT的实际参考; T＆GT; 为的IList 指针;并使用名单，其中，T＆GT; 的insteed 的IList＆LT; T＆GT; 实际上是一个不好的编程习惯，不服务于所有目的。

Moreover using IList<object> is obviously not the solution because without out modifier generics can not be assigned to a less inherit class; and creating new instance of List is not a solution here because someone may want actual reference of the IList<T> as an IList pointer; and use List<T> insteed of IList<T> is actually a bad programming practice and doesn't serve all purpose.

如果.NET想给灵活性，每个实施的IList＆LT的; T＆GT; 不应该有非一般的实现合同（即的IList ），那么他们为什么不保持它们实现通用和非通用版本，并没有表明其希望所有的具体类合同的通用和非遗传项通过，应该收缩的另一个接口接口。

If .NET wants to give flexibility that every implementation of IList<T> should not have a contract of non-generic implementation (i.e. IList) then why they didn't keep another interface which implement both generic and non-generic version and didn't suggest that all concrete class which want to contract for generic and non-genetic item should contract via that interface.

同样的问题出现铸造的ICollection＆LT; T＆GT; 到的ICollection 和的IDictionary＆LT; TKEY的，TValue＆GT; 到的IDictionary

Same problem occurs for casting ICollection<T> to ICollection and IDictionary<TKey, TValue> to IDictionary.

推荐答案

正如您注意， T 在的IList＆LT; T＆GT; 不是的协的。作为一个经验法则：任何类可以修改它的状态不能协变的。其原因是这样的类通常具有有方法 T 作为自己的参数，例如中的一个的类型无效添加（T元素）。而协变型参数中不允许的输入的位置。

As you note, T in IList<T> is not covariant. As a rule of thumb: any class that can modify its state cannot be covariant. The reason is that such classes often have methods that have T as the type of one of their parameters, e.g. void Add(T element). And covariant type parameters are not allowed in input positions.

泛型增加了，除其他原因外，提供类型安全。例如，你不能添加象到苹果的列表。如果的ICollection＆LT; T＆GT; 要延长的ICollection ，那么你可以调用（（ICollection的） myApples）。新增（someElephant）没有编译时错误，因为的ICollection 有一个方法无效添加（obj对象），它似乎允许您添加的任意的对象名单，而在实践中只能添加 T的对象。因此，的ICollection＆LT; T＆GT; 不延的ICollection 和的IList＆LT; T＆GT; 不延的IList 。

Generics were added, among other reasons, to provide type safety. For example, you can't add an Elephant to a list of Apple. If ICollection<T> were to extend ICollection, then you could call ((ICollection)myApples).Add(someElephant) without a compile-time error, as ICollection has a method void Add(object obj), which seemingly allows you to add any object to the list, while in practice you can only add objects of T. Therefore, ICollection<T> does not extend ICollection and IList<T> does not extend IList.

安德斯·海尔斯伯格，C＃的创造者之一，解释像这样：

Anders Hejlsberg, one of the creators of C#, explains it like this:

在理想情况下所有的泛型集合接口（如ICollection的，IList的）会从他们的非通用同行继承，使得通用接口实例可以与通用和非通用code同时使用。

Ideally all of the generic collection interfaces (e.g. ICollection, IList) would inherit from their non-generic counterparts such that generic interface instances could be used both with generic and non-generic code.

事实证明，唯一的通用接口的量，这是可能的是IEnumerable的，因为只有IEnumerable的是禁忌的变体：在IEnumerable的，所述类型参数T仅用于输出的位置（返回值），而不是在输入位置（参数）。的ICollection和IList的使用T在输入和输出的位置，并且这些接口，因此不变。

As it turns out, the only generic interface for which this is possible is IEnumerable, because only IEnumerable is contra-variant: In IEnumerable, the type parameter T is used only in "output" positions (return values) and not in "input" positions (parameters). ICollection and IList use T in both input and output positions, and those interfaces are therefore invariant.

由于净4.5存在的 IReadOnlyCollection＆LT;出T＆GT; 和 IReadOnlyList＆LT;出T＆GT; 协接口。但的IList＆LT; T＆GT; ，的ICollection＆LT; T＆GT; 和许多列表和集合类不实行或延长他们。坦率地说，我觉得他们不是非常有用，因为它们只定义计数和此[INT指数] 。

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Since .Net 4.5 there are the IReadOnlyCollection<out T> and IReadOnlyList<out T> covariant interfaces. But IList<T>, ICollection<T> and many of the list and collection classes don't implement or extend them. Frankly, I find them not very useful, as they only define Count and this[int index].

如果我能重新设计的.Net 4.5，从地上爬了起来，我会分裂列表界面为只读协接口的IList＆LT;出T＆GT; ，包括包含和的IndexOf ，和一个可变不变的接口 IMutableList＆LT; T＆GT; 。然后，你可以施放的IList＆LT;苹果＆GT; 到的IList＆LT;对象＆gt; 。我实现了这个位置：

If I could redesign .Net 4.5 from the ground up, I would have split the list interface into a read-only covariant interface IList<out T> that includes Contains and IndexOf, and a mutable invariant interface IMutableList<T>. Then you could cast IList<Apple> to IList<object>. I implemented this here:

M42集合 - 协变的集合，列表和数组。

M42 Collections - Covariant collections, lists and arrays.

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