Parallel.ForEach() vs. foreach(IEnumerable<T>.AsParallel())

SnickersAreMyFave picture SnickersAreMyFave · Sep 24, 2010 · Viewed 70k times · Source

Erg, I'm trying to find these two methods in the BCL using Reflector, but can't locate them. What's the difference between these two snippets?

A:

IEnumerable<string> items = ...

Parallel.ForEach(items, item => {
   ...
});

B:

IEnumerable<string> items = ...

foreach (var item in items.AsParallel())
{
   ...
}

Are there different consequences of using one over the other? (Assume that whatever I'm doing in the bracketed bodies of both examples is thread safe.)

Answer

user180326 picture user180326 · Sep 24, 2010

They do something quite different.

The first one takes the anonymous delegate, and runs multiple threads on this code in parallel for all the different items.

The second one not very useful in this scenario. In a nutshell it is intended to do a query on multiple threads, and combine the result, and give it again to the calling thread. So the code on the foreach statement stays always on the UI thread.

It only makes sense if you do something expensive in the linq query to the right of the AsParallel() call, like:

 var fibonacciNumbers = numbers.AsParallel().Select(n => ComputeFibonacci(n));