一些帮助理解和QUOT;产量＆QUOT; [英] Some help understanding "yield"

问题描述

在我永恒的追求，少犯几次我一直撞到了收益的语句。

我试图总结我的头周围现在几次，但我难倒具有相同的错误每次。

In my everlasting quest to suck less I keep bumping into the "yield" statement.
I have tried to wrap my head around it a few times now, but I'm stumped every time with the same error.

它去有点像这样：

的[的someMethod]身体不能是迭代器块，因为
  System.Collections.Generic.List＆LT; ACLASS>'不是一个迭代器接口类型。

The body of [someMethod] cannot be an iterator block because 'System.Collections.Generic.List< AClass>' is not an iterator interface type.

这是code，我最后卡住了：

This is the code where I last got stuck:

        foreach (XElement header in headersXml.Root.Elements()){
            yield return (ParseHeader(header));                
        }

我是什么做错了吗？我不能用在一个迭代的收益呢？那么什么意义呢？
在这种情况下例如，它说，名单＆LT; ProductMixHeader>不是一个迭代器接口类型。
ProductMixHeader是一个自定义类，但我想列表是一个迭代器接口类型，不？

What am I doing wrong? Can't I use the yield in an iterator? Then what's the point? In the case example it said that List< ProductMixHeader> is not an iterator interface type. ProductMixHeader is a custom class, but I imagine List is an iterator interface type, not?

- 编辑 -
感谢所有的快速解答。答案给一个答案。
我知道这个问题是不是所有的新的和相同的resouces不断弹出。
原来，我想我可以回到列表作为返回类型，但脱离当然，因为列表不懒，也不能使用。改变我的返回类型IEnumerable的解决了这个问题：D

--Edit-- Thanks for all the quick answers. Answer given to first answer. I know this question isn't all that new and the same resouces keep popping up. It turned out I was thinking I could return List as a return type, but off course because List isn't lazy, it cannot be used. Changing my return type to IEnumerable solved the problem :D

现在有点相关的问题（不值得打开一个新的线程）：这是值得给予了IEnumerable作为返回类型，如果我敢肯定，我要去.ToList（）反正99％的情况？什么会为我做的表现吗？

Now a somewhat related question (not worth opening a new thread): is it worth giving IENumerable as a return type if I'm sure that 99% of the cases I'm going to go .ToList() anyway? What will performance do for me there?

推荐答案

使用方法的收益回报率的必须声明为返回以下两个接口之一：

A method using yield return must be declared as returning one of the following two interfaces:

IEnumerable<SomethingAppropriate>
IEnumerator<SomethingApropriate>

（感谢乔恩并的马克获取指出的IEnumerator）

(thanks Jon and Marc for pointing out IEnumerator)

例如：

public IEnumerable<AClass> YourMethod()
{
    foreach (XElement header in headersXml.Root.Elements())
    {
        yield return (ParseHeader(header));                
    }
}

收益率数据的懒惰生产，只有生产其他项目后的第一个被检索，而返回一个列表将一气呵成返回的一切。

yield is a lazy producer of data, only producing another item after the first has been retrieved, whereas returning a list will return everything in one go.

所以是有区别的，你需要正确地申报方法。

So there is a difference, and you need to declare the method correctly.

有关更多信息，请阅读乔恩的回答这里，其中包含了一些非常有用的链接

For more information, read Jon's answer here, which contains some very useful links.

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