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

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));

The second method will not be parallel the correct way to use AsParallel() in your example would be

IEnumerable<string> items = ...items.AsParallel().ForAll(item =>{    //Do parallel stuff here});

The difference is, B isn't parallel. The only thing AsParallel() does is that it wraps around a IEnumerable, so that when you use LINQ methods, their parallel variants are used. The wrapper's GetEnumerator() (which is used behind the scenes in the foreach) even returns the result of the original collection's GetEnumerator().

BTW, if you want to look at the methods in Reflector, AsParallel() is in the System.Linq.ParallelEnumerable class in the System.Core assembly. Parallel.ForEach() is in the mscorlib assembly (namespace System.Threading.Tasks).