为什么我会得到调用函数功能:LT时错误的结果; INT> ;? [英] Why am I getting wrong results when calling Func<int>?
问题描述
我有下面的代码片段在C#中:
I have the following code snippet in C#:
var actions = new List<Func<int>>();
IEnumerable<int> values = new List<int> { 1, 2, 3 };
foreach (int value in values)
{
actions.Add(() => value * value);
}
foreach (var action in actions)
{
Console.WriteLine(action()); ;
}
Console.ReadLine();
它的运行很好,但我没有得到结果我期望的那样。
It's running fine, but I am not getting the result I expect.
实际结果
9,9,9
预期结果
1,4,9
为什么我不能得到结果我期望?
Why am I not getting the result I expect?
推荐答案
您正在捕捉你的lambda表达式中的循环变量,这意味着当代表终于调用的,它看到的最后的循环变量的值
You're capturing the loop variable within your lambda expression, which means when the delegate is finally invoked, it's seeing the final value of the loop variable.
简单的修正:
foreach (int value in values)
{
int copy = value;
actions.Add(() => copy * copy);
}
这样,你得到一个新的复制
在循环的每次迭代变量,所以每位代表表达式将捕捉不同的变量,而且他们没有受到循环变量(值
)切换时间。
This way you get a new copy
variable in each iteration of the loop, so each delegate expression will capture a different variable, and they're not affected by the loop variable (value
) changing over time.
埃里克利珀解释了这个井的关闭了被认为是有害的循环变量(和的part 2 )。
Eric Lippert explains this well in "Closing over the loop variable considered harmful" (and part two).
基本上这是一个在C疑难杂症#这几乎每个人都爱上了迟早的事情。
Basically this is a "gotcha" in C# which nearly everyone falls for sooner or later.
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