Parallel.Foreach vs Foreach and Task in local variable

Question

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?

As of C# 5, actually the local variable is unnecessary as loop variables are semantically inside the loop. — Mike Zboray
– Mike Zboray, Commented Apr 6, 2013 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
– Saeid, Commented Apr 6, 2013 at 6:01
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 Zboray
– Mike Zboray, Commented Apr 6, 2013 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
– Gennady Vanin Геннадий Ванин, Commented Apr 6, 2013 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
– Saeid, Commented Apr 6, 2013 at 12:55

Jean Hominal · Accepted Answer · 2013-04-06 11:32:24Z

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.

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