current community

your communities

more stack exchange communities

Stack Exchange
tour help
careers 2.0
Take the 2-minute tour ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free, no registration required.
up vote 1 down vote favorite
3

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());

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

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

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?

UPDATE

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

share|improve this question
    
As of C# 5, actually the local variable is unnecessary as loop variables are semantically inside the loop. –  mike z Apr 6 '13 at 5:52
    
@mikez What do you mean of loop? you mean both foreach and Parallel.foreach? if yes? so how about .NET 4? –  Saeid Apr 6 '13 at 6:01
1  
In C# 5, the local variable is unnecessary in the foreach loop (I'd say Parallel.ForEach is a method call not a loop). In C# 4 it is necessary. I would also note this is compiler behavior, not runtime behavior. This is a "breaking" change from previous compiler versions. See blogs.msdn.com/b/ericlippert/archive/2009/11/12/… –  mike z Apr 6 '13 at 6:15
    
Please inform me what is so heavy wrong in my answer that it was so hevy downvoted. I am really curious –  Gennady Vanin Геннадий Ванин Apr 6 '13 at 12:01
    
@Gennady Vanin -- Novosibirsk I don't know why the others down voted, but in my case: First: .Net 4.5 tagged because I use .Net 4.5 Second: both versions are correct none of them is not wrong, Third: your description is so ambiguous and the code you pasted why use Parallel.foreach and Task and Taskpool together, what advantages you followed? and also in your Update: there not the same result. thanks –  Saeid Apr 6 '13 at 12:55

1 Answer 1

up vote 2 down vote accepted

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.

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.

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));
    }
}

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.

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

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.