IEnumerable<T>作为返回类型 [英] IEnumerable&lt;T&gt; as return type

问题描述

使用 IEnumerable 作为返回类型有问题吗?FxCop 抱怨返回 List(它建议返回 Collection 代替).

Is there a problem with using IEnumerable<T> as a return type? FxCop complains about returning List<T> (it advises returning Collection<T> instead).

嗯，我一直遵循这样一条规则:接受最少的，但返回最多的."

Well, I've always been guided by a rule "accept the least you can, but return the maximum."

从这个角度来说，返回IEnumerable是一件坏事，但是想使用懒惰检索"怎么办呢?此外，yield 关键字非常有用.

From this point of view, returning IEnumerable<T> is a bad thing, but what should I do when I want to use "lazy retrieval"? Also, the yield keyword is such a goodie.

推荐答案

这实际上是一个由两部分组成的问题.

This is really a two part question.

1) 返回一个 IEnumerable

1) Is there inherently anything wrong with returning an IEnumerable<T>

什么都没有.事实上，如果您使用 C# 迭代器，这就是预期的行为.将其转换为 List或者先发制人的另一个集合类不是一个好主意.这样做是对调用者的使用模式做出假设.我发现对来电者的任何假设都不是一个好主意.他们可能有充分的理由想要一个 IEnumerable.也许他们想将其转换为完全不同的集合层次结构(在这种情况下，转换为 List 就浪费了).

No nothing at all. In fact if you are using C# iterators this is the expected behavior. Converting it to a List<T> or another collection class pre-emptively is not a good idea. Doing so is making an assumption on the usage pattern by your caller. I find it's not a good idea to assume anything about the caller. They may have good reasons why they want an IEnumerable<T>. Perhaps they want to convert it to a completely different collection hierarchy (in which case a conversion to List is wasted).

2) 是否有任何情况下返回 IEnumerable 以外的内容可能更可取?

2) Are there any circumstances where it may be preferable to return something other than IEnumerable<T>?

是的.虽然对呼叫者假设太多并不是一个好主意，但根据自己的行为做出决定是完全可以的.想象一个场景，您有一个多线程对象，它将请求排队到一个不断更新的对象中.在这种情况下，返回原始 IEnumerable<T>是不负责任的.一旦集合被修改，可枚举将失效并导致执行.相反，您可以拍摄结构的快照并返回该值.在列表中说形式.在这种情况下，我只会将对象作为直接结构(或接口)返回.

Yes. While it's not a great idea to assume much about your callers, it's perfectly okay to make decisions based on your own behavior. Imagine a scenario where you had a multi-threaded object which was queueing up requests into an object that was constantly being updated. In this case returning a raw IEnumerable<T> is irresponsible. As soon as the collection is modified the enumerable is invalidated and will cause an execption to occur. Instead you could take a snapshot of the structure and return that value. Say in a List<T> form. In this case I would just return the object as the direct structure (or interface).

不过这种情况肯定很少见.

This is certainly the rarer case though.

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