多线程和lambda变量的作用域 [英] multi-threading and lambda variable scope

问题描述

我使用的管理线程来运行操作的池类。它最初被编码为采取行动（不带参数），我在呼唤这样的：

I am using a class that manages a pool of threads to run actions. Originally it was coded to take an Action (with no parameter) and I was calling it like this:

void LoadTasks()
{
   string param;

   // some code loops and changes param
   {
      threadPool.EnqueueTask(() => SomeMethod(param));
   }
}

由于线程跑一些是不错，但偶尔参数变量是不是我所期待......这是一个新值，而不是我故意要发送到的方法。

As the threads ran some were fine, but occasionally the param variable was not what I expected ... it was a "newer" value and not what I intended to send to the method.

更改线程池来接受动作<对象> 并调用没有一个lambda - 这样 threadPool.EnqueueTask（的someMethod，参数） - 工作围绕我的问题。

Changing the thread pool to accept Action<Object> and calling without a lambda -- like this threadPool.EnqueueTask(SomeMethod, param) -- worked around my problem.

我看到有关线程安全的C＃lambda表达式不少问题。例如， lambda表达式的一个公认的答案的就不太可能是线程安全比你期望的那样。的我发现大约lambda表达式/关闭/范围界定等问题和答案是混乱。所以我在寻找lambda表达式和可变范围的解释，最好与在我的例子的问题。

I see quite a few questions about C# lambdas about thread-safety. For example, an accepted answer of lambdas are much less likely to be thread safe than you would expect. I'm finding other questions and answers about lambdas/closures/scoping to be confusing. So I am looking for an explanation of lambdas and variable scope, ideally relating to the problem in my example.

推荐答案

所以，问题是，你在关闭，你不是说要变量。最简单的修补程序是最重要的情况下，是创建一个新的局部变量，复制你曾经关闭过变量，然后关上的是的代替。

So the problem is that you're closing over variables that you don't mean to. The easy fix in most all cases is to create a new local variable, copy the variable you were once closing over, and then close over that instead.

因此，而不是：

for(int i = 0; i < number; i++)
{
    threadPool.EnqueueTask(() => SomeMethod(someList[i]));
}

您可以这样做：

for(int i = 0; i < number; i++)
{
    int copy = i;
    threadPool.EnqueueTask(() => SomeMethod(someList[copy]));
}

现在每个拉姆达被关闭了它自己的变量，而不是所有的人关过的同一个变量。

Now each lambda is closing over it's own variable, rather than having all of them close over the same one variable.

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