Parallel.Foreach与Foreach和局部变量中的Task [英] Parallel.Foreach vs Foreach and Task in local variable

问题描述

当我们使用foreach和Tasks时，我们需要使用这样的局部变量:

When we use foreach and Tasks we need to use local variables like this:

List<Task> TaskPool = new List<Task>();
foreach (TargetType Item in Source)
{
  TargetType localItem = Item;
  TaskPool.Add(Task.Factory.StartNew(() => DoSomething(localItem)));
}
Task.WaitAll(TaskPool.ToArray());

但是Parallel.Foreach呢，我像这样使用它:

But how about Parallel.Foreach, I use it like this:

Parallel.ForEach(Source, (TargetType item) => DoSomething(item));

因此，您看不到任何局部变量.但是Parallel.Foreach如何工作?不需要引入任何局部变量吗?或者，如果需要的话，我该如何定义它?

So there is not any Local Variable as you see. But how does Parallel.Foreach work? Is there no need to introduce any local variables? or if needed, how can I define it?

更新

.NET 4和.NET 4.5有什么区别吗?

Is there any difference in .NET 4 and .NET 4.5?

推荐答案

您没有在Parallel.ForEach中定义任何局部变量-item仅仅是形式参数-Parallel.ForEach的实现代码就是一个必须处理变量，以及变量是局部变量，捕获变量还是其他变量.

You do not define any local variable in Parallel.ForEach - item is nothing more than a formal parameter - the implementation code of Parallel.ForEach is the one that will have to handle variables, and whether they are local, captured or something else.

无需定义与形式参数有关的局部变量Parallel.ForEach-您的匿名委托的调用者代码将处理该变量并将其传递给您的函数.

There is no need to define a local variable related to the formal parameter Parallel.ForEach - the caller code of your anonymous delegate will handle the variable and pass it to your function.

但是在C#4中，如果捕获另一个变量，则可能需要使用局部变量，即:

However in C#4, you might need to use a local variable if you capture another variable, that is:

void DoSomething(ItemType item, OtherType other) {
}

void YourFunction(IEnumerable<ItemType> items, IEnumerable<OtherType> others) {

    foreach (var otherItem in others) {
        var localOtherItem = otherItem;
        Parallel.ForEach(items, item => DoSomething(item, localOtherItem));
    }
}

您可以看到上面的区别:localOtherItem来自定义了匿名函数的上下文:称为闭包.而items中的项目仅作为方法参数传递给匿名函数.

You can see the difference above: localOtherItem is taken from the context where the anonymous function is defined: that is called a closure. Whereas the items in items are passed simply as a method parameter to the anonymous function.

简而言之:Parallel.ForEach中的item和C#foreach中的item是两个非常不同的问题.

In short: the item in Parallel.ForEach and the item in C# foreach are two very different problems.

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