ReSharper的：IEnumerable的的可能的多个枚举 [英] Resharper: Possible Multiple Enumeration of IEnumerable

问题描述

我使用的是新版本的ReSharper的6.在我的代码好几个地方已经强调了一些文本，并警告我，有可能是IEnumerable的的的可能多枚举的

I'm using the new Resharper version 6. In several places in my code it has underlined some text and warned me that there may be a Possible multiple enumeration of IEnumerable.

我明白这意味着什么，并采取建议在适当情况下，但在某些情况下，我不知道它实际上是一个大问题。

I understand what this means, and have taken the advice where appropriate, but in some cases I'm not sure it's actually a big deal.

就像下面的代码：

var properties = Context.ObjectStateManager.GetObjectStateEntry(this).GetModifiedProperties();
if (properties.Contains("Property1") || properties.Contains("Property2") || properties.Contains("Property3")) {
    ...
}

它强调属性的每一个提到的第二行警告说，我列举了这IEnumerable的多次。

It's underlining each mention of properties on the second line, warning that I am enumerating over this IEnumerable multiple times.

如果我想补充 .ToList（）来结束1号线（转动属性从的IEnumerable<串> 到列表<串> ）的警告消失。

If I add .ToList() to the end of line 1 (turning properties from a IEnumerable<string> to a List<string>), the warnings go away.

但可以肯定，如果我把它转换成一个列表，然后它会枚举在整个IEnumerable的建立列表中的第一个地方，然后枚举名单按要求找到的属性（即1个整体枚举和3部分枚举）。而在我的原代码，它只是在做局部3枚举。

But surely, if I convert it to a List, then it will enumerate over the entire IEnumerable to build the List in the first place, and then enumerate over the List as required to find the properties (i.e. 1 full enumeration, and 3 partial enumerations). Whereas in my original code, it is only doing the 3 partial enumerations.

难道我错了吗？什么是这里最好的方法是什么？

Am I wrong? What is the best method here?

推荐答案

我不知道究竟你的属性真的是在这里 - 但如果它本质上代表一个unmaterialized数据库查询，那么你的如果语句将执行三个查询

I don't know exactly what your properties really is here - but if it's essentially representing an unmaterialized database query, then your if statement will perform three queries.

我的犯罪嫌疑人的这将是更好的事：

I suspect it would be better to do:

string[] propertiesToFind = { "Property1", "Property2", "Property3" };
if (properties.Any(x => propertiesToFind.Contains(x))
{
     ...
}

这会的逻辑的只能通过序列重复一次 - 如果有涉及到数据库查询，这很可能是能够只使用一个SQLIN的条款做这一切在一个查询数据库。

That will logically only iterate over the sequence once - and if there's a database query involved, it may well be able to just use a SQL "IN" clause to do it all in the database in a single query.

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