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.)
The difference is, B isn't parallel. The only thing
AsParallel()
does is that it wraps around aIEnumerable
, so that when you use LINQ methods, their parallel variants are used. The wrapper'sGetEnumerator()
(which is used behind the scenes in theforeach
) even returns the result of the original collection'sGetEnumerator()
.BTW, if you want to look at the methods in Reflector,
AsParallel()
is in theSystem.Linq.ParallelEnumerable
class in theSystem.Core
assembly.Parallel.ForEach()
is in themscorlib
assembly (namespaceSystem.Threading.Tasks
).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:The second method will not be parallel the correct way to use AsParallel() in your example would be