闭包在 for 和 foreach 循环中的行为不同 [英] Closures behaving differently in for and foreach loops

问题描述

在 C# 中尝试闭包时，我发现如果它们在循环中捕获迭代器变量，它们会意外地工作.

While experimenting with closures in C# I found out that they work rather unexpectedly if they capture an iterator variable in a loop.

var actions = new List<Action>();

foreach (int i in new[] { 1, 2 })
    actions.Add(() => Console.WriteLine(i));

for (int i = 3; i <= 4; i++)
    actions.Add(() => Console.WriteLine(i));

foreach (var action in actions)
    action();

上面的代码产生了一个奇怪的结果(我使用的是 .NET 4.5 编译器):

The above code produces a strange result (I'm using .NET 4.5 compiler):

1
2
5
5

为什么 i 的值在 2 个几乎相同的循环中的捕获方式不同?

Why is the value of i captured differently for 2 almost identical loops?

推荐答案

在 C# 5 及更高版本中，foreach 循环声明了一个 separate i 循环每次迭代的变量.因此，每个闭包都捕获一个单独的变量，您会看到预期的结果.

In C# 5 and beyond, the foreach loop declares a separate i variable for each iteration of the loop. So each closure captures a separate variable, and you see the expected results.

在 for 循环中，你只有一个 single i 变量，它被所有的闭包捕获，并修改为循环进展 - 因此当您调用委托时，您会看到该单个变量的最终值.

In the for loop, you only have a single i variable, which is captured by all the closures, and modified as the loop progresses - so by the time you call the delegates, you see the final value of that single variable.

在 C# 2、3 和 4 中，foreach 循环的行为也是如此，这基本上从不所需的行为，因此它在 C# 5 中得到了修复.

In C# 2, 3 and 4, the foreach loop behaved that way as well, which was basically never the desired behaviour, so it was fixed in C# 5.

在for循环中，如果在循环体的范围内引入一个新的变量，也可以达到同样的效果:

You can achieve the same effect in the for loop if you introduce a new variable within the scope of the loop body:

for (int i = 3; i <= 4; i++)
{
    int copy = i;
    actions.Add(() => Console.WriteLine(copy));
}

有关更多详细信息，请阅读 Eric Lippert 的博客文章关闭循环变量被认为有害" - 第 1 部分，第 2 部分.

For more details, read Eric Lippert's blog posts, "Closing over the loop variable considered harmful" - part 1, part 2.

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