如何在LINQ的`where`子句中更新全局变量? [英] How to update a global variable inside `where` clause in LINQ?

问题描述

我想使用 LINQ 和 Where 扩展方法来过滤列表.但是除了过滤之外，我还想更新 Where 中的全局变量.但是我做不到.考虑以下示例:

I want to filter a list using LINQ with Where extension method. But apart from filtering I also want to update a global variable inside Where. However I cannot do it. Consider this example:

var list = new List<string> { "1", "2", "3", "4", "5" };

bool flag = false;
var newList = list.Where(item =>
{
    flag = true;
    return item == "2";
});

// Here I expect flag = true, but in fact it's false
Console.Write(flag);

如您所见，我将 flag = true 设置为，执行后仍将值 flag == false 设置为

.这对我来说没有意义.您能解释一下幕后发生的事情以及为什么 flag 不变的原因.还有没有办法完全更改 LINQ 内部的全局变量?

As you can see I set flag = true, still the value flag == false after execution. It does not make sense to me. Can you explain what is going on under the hood and why flag is not changed. Also is there a way to change global variables inside LINQ at all?

推荐答案

Linq查询是惰性的，因此在枚举newList之前，您将看不到更改，因为您的位置尚未执行.

Linq queries are lazy, so until you enumerate newList you will not see a change, because your where has not been executed.

var list = new List<string> { "1", "2", "3", "4", "5" };

bool flag = false;
var newList = list.Where(item =>
{
    flag = true;
    return item == "2";
});

Console.WriteLine(flag); // Flag is still false.

foreach (var item in newList) {
  // It doesn't matter what we do here, just that we enumerate the list.
}

Console.Write(flag); // Flag is now true.

foreach导致执行位置并设置标志.

The foreach causes the where to execute and sets your flag.

我真的建议您不要使用where谓词来产生副作用，但这是您要这样做的方式.

I would really advise against using the where predicate to create a side effect, by the way, but this is how you'd do it.

这篇关于如何在LINQ的`where`子句中更新全局变量?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！