How do I collect return values from Parallel.ForEach?

Question

I'm calling a slow webservice in parallel. Things were great until I realized I need to get some information back from the service. But I don't see where to get the values back. I can't write to the database, HttpContext.Current appears to be null inside of a method called using Parallel.ForEach

Below is a sample program (in your mind, please imagine a slow web service instead of a string concatenation)

using System; using System.Threading.Tasks;  class Program {     static void Main(string[] args)     {         WordMaker m = new WordMaker();         m.MakeIt();     }     public class WordMaker     {         public void MakeIt()         {             string[] words = { "ack", "ook" };             ParallelLoopResult result = Parallel.ForEach(words, word => AddB(word));             Console.WriteLine("Where did my results go?");             Console.ReadKey();         }         public string AddB(string word)         {             return "b" + word;         }     }  } 

Answer 1

You've discarded it in here.

ParallelLoopResult result = Parallel.ForEach(words, word => AddB(word)); 

You probably want something like,

ParallelLoopResult result = Parallel.ForEach(words, word => {     string result = AddB(word);     // do something with result }); 

If you want some sort of collection at the end of this, consider using one of the collections under System.Collections.Concurrent, like ConcurrentBag

ConcurrentBag<string> resultCollection = new ConcurrentBag<string>(); ParallelLoopResult result = Parallel.ForEach(words, word => {     resultCollection.Add(AddB(word)); });  // Do something with the result 

Answer 2

Your may consider using AsParallel extension method of IEnumerable, it will take care of the concurrency for you and collect the results.

words.AsParallel().Select(AddB).ToArray()

Synchronisation (e.g. locks or concurrent collections that use locks) are usually bottleneck of concurrent algorithms. The best is to avoid synchronisation as much as possible. I am guessing that AsParallel uses something smarter like putting all the items produced on single thread into a local non-concurrent collection and then combining these at the end.