問(wèn)題描述

在我不斷追求減少吸吮的過(guò)程中，我試圖理解yield"語(yǔ)句，但我一直遇到同樣的錯(cuò)誤.

In my everlasting quest to suck less I'm trying to understand the "yield" statement, but I keep encountering the same error.

[someMethod] 的主體不能是迭代器塊，因?yàn)?System.Collections.Generic.List<AClass>' 不是迭代器接口類(lèi)型.

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

這是我卡住的代碼:

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

我做錯(cuò)了什么?我不能在迭代器中使用 yield 嗎?那有什么意義呢?在此示例中，它表示 List<ProductMixHeader> 不是迭代器接口類(lèi)型.ProductMixHeader 是一個(gè)自定義類(lèi)，但我想 List 是一個(gè)迭代器接口類(lèi)型，不是嗎?

What am I doing wrong? Can't I use yield in an iterator? Then what's the point? In this 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, no?

--編輯--
感謝所有快速回答.
我知道這個(gè)問(wèn)題并不是那么新鮮，并且不斷出現(xiàn)相同的資源.
原來(lái)我在想我可以返回 List<AClass> 作為返回類(lèi)型，但由于 List<T> 不是惰性的，它不能.將我的返回類(lèi)型更改為 IEnumerable<T> 解決了問(wèn)題:D

--Edit--
Thanks for all the quick answers.
I know this question isn't all that new and the same resources keep popping up.
It turned out I was thinking I could return List<AClass> as a return type, but since List<T> isn't lazy, it cannot. Changing my return type to IEnumerable<T> solved the problem :D

一個(gè)有點(diǎn)相關(guān)的問(wèn)題(不值得打開(kāi)一個(gè)新線程):如果我確定 99% 的情況我要去，是否值得將 IEnumerable<T> 作為返回類(lèi)型去 .ToList() 反正?性能影響是什么?

A somewhat related question (not worth opening a new thread): is it worth giving IEnumerable<T> as a return type if I'm sure that 99% of the cases I'm going to go .ToList() anyway? What will the performance implications be?

推薦答案

使用yield return的方法必須聲明為返回以下兩個(gè)接口之一:

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

IEnumerable<SomethingAppropriate>
IEnumerator<SomethingApropriate>

(感謝 喬恩 和 Marc 用于指出 IEnumerator)

(thanks Jon and Marc for pointing out IEnumerator)

例子:

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

yield 是一個(gè)懶惰的數(shù)據(jù)生產(chǎn)者，僅在檢索到第一個(gè)項(xiàng)目后才生成另一個(gè)項(xiàng)目，而返回列表將一次性返回所有內(nèi)容.

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.

所以有區(qū)別，需要正確聲明方法.

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

有關(guān)更多信息，請(qǐng)閱讀 Jon 的答案，其中包含一些非常有用的鏈接.

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

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