Resharper：可能的多个枚举的IEnumerable(Resharper: Possible Multiple Enumeration of IEnumerable)

Resharper：可能的多个枚举的IEnumerable(Resharper: Possible Multiple Enumeration of IEnumerable)

我正在使用新的Resharper版本6.在我的代码中的几个地方，它强调了一些文本，并警告我可能会有一个可能的多次枚举的IEnumerable 。

我明白这是什么意思，并在适当的时候采取了建议，但在某些情况下，我不知道这实际上是一个大问题。

像下面的代码一样：

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

它强调了每一次提到的properties在第二行，警告我列举了这个IEnumerable多次。

如果将.ToList()添加到行1的末尾（将properties从IEnumerable<string>转换为List<string> ），则警告消失。

但是肯定的是，如果我把它转换成一个List，那么它将枚举整个IEnumerable来构建List，然后根据需要枚举List，以查找属性（即1个完整的枚举和3个部分枚举）。 而在我的原始代码中，它只是做了3个部分枚举。

我错了吗？ 这里最好的方法是什么？

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.

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.

Like in the following code:

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

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

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

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.

Am I wrong? What is the best method here?

最满意答案

我不知道你的properties真的在这里 - 但是如果它本质上代表一个非物质化的数据库查询，那么你的if语句将执行三个查询。

我怀疑这样做会更好：

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

这将在逻辑上仅迭代序列一次 - 如果涉及到数据库查询，则可能只需使用SQL“IN”子句在单个查询中的数据库中完成所有操作。

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.

I suspect it would be better to do:

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

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.