C# 中的数组如何部分实现 IList<T>? [英] How do arrays in C# partially implement IList<T>?

问题描述

如您所知，C# 中的数组实现了 IList 以及其他接口.但是不知何故，他们在没有公开实现 IList 的 Count 属性的情况下做到了这一点！数组只有一个 Length 属性.

So as you may know, arrays in C# implement IList<T>, among other interfaces. Somehow though, they do this without publicly implementing the Count property of IList<T>! Arrays have only a Length property.

这是 C#/.NET 违反自己的关于接口实现的规则的公然例子还是我遗漏了什么?

Is this a blatant example of C#/.NET breaking its own rules about the interface implementation or am I missing something?

推荐答案

根据汉斯的回答新的答案

感谢 Hans 给出的答案，我们可以看到实现比我们想象的要复杂一些.编译器和 CLR 都非常努力试图给人一种数组类型实现了 IList 的印象——但是数组变化使这变得更加棘手.与 Hans 的回答相反，数组类型(无论如何都是一维的，从零开始)确实直接实现了泛型集合，因为任何特定数组的类型不是 System.Array - 这只是数组的 base 类型.如果你问一个数组类型它支持什么接口，它包括泛型类型:

Thanks to the answer given by Hans, we can see the implementation is somewhat more complicated than we might think. Both the compiler and the CLR try very hard to give the impression that an array type implements IList<T> - but array variance makes this trickier. Contrary to the answer from Hans, the array types (single-dimensional, zero-based anyway) do implement the generic collections directly, because the type of any specific array isn't System.Array - that's just the base type of the array. If you ask an array type what interfaces it supports, it includes the generic types:

foreach (var type in typeof(int[]).GetInterfaces())
{
    Console.WriteLine(type);
}

输出:

System.ICloneable
System.Collections.IList
System.Collections.ICollection
System.Collections.IEnumerable
System.Collections.IStructuralComparable
System.Collections.IStructuralEquatable
System.Collections.Generic.IList`1[System.Int32]
System.Collections.Generic.ICollection`1[System.Int32]
System.Collections.Generic.IEnumerable`1[System.Int32]

对于一维、从零开始的数组，就语言而言，该数组也确实实现了IList.C# 规范的第 12.1.2 节是这样说的.因此，无论底层实现做什么，语言都必须表现，就好像 T[] 的类型实现了 IList界面.从这个角度来看，接口是实现的，其中一些成员被显式实现(例如Count).这是语言层面对正在发生的事情的最好解释.

For single-dimensional, zero-based arrays, as far as the language is concerned, the array really does implement IList<T> too. Section 12.1.2 of the C# specification says so. So whatever the underlying implementation does, the language has to behave as if the type of T[] implements IList<T> as with any other interface. From this perspective, the interface is implemented with some of the members being explicitly implemented (such as Count). That's the best explanation at the language level for what's going on.

请注意，这仅适用于一维数组(和基于零的数组，而不是 C# 作为一种语言对非基于零的数组有任何说明).T[,] 没有实现IList.

Note that this only holds for single-dimensional arrays (and zero-based arrays, not that C# as a language says anything about non-zero-based arrays). T[,] doesn't implement IList<T>.

从 CLR 的角度来看，一些更时髦的事情正在发生.您无法获得通用接口类型的接口映射.例如:

From a CLR perspective, something funkier is going on. You can't get the interface mapping for the generic interface types. For example:

typeof(int[]).GetInterfaceMap(typeof(ICollection<int>))

例外:

Unhandled Exception: System.ArgumentException: Interface maps for generic
interfaces on arrays cannot be retrived.

那为什么奇怪呢?好吧，我相信这真的是由于数组协方差，这是类型系统 IMO 中的一个问题.即使 IList 不是协变(并且不能安全)，数组协变允许这样做:

So why the weirdness? Well, I believe it's really due to array covariance, which is a wart in the type system, IMO. Even though IList<T> is not covariant (and can't be safely), array covariance allows this to work:

string[] strings = { "a", "b", "c" };
IList<object> objects = strings;

...这使得它看起来就像 typeof(string[]) 实现了 IList