Question

When I use Parallel.ForEach or Parallel.For in the System.Threading.Tasks namespace, can I assume that all threads are synchronised before execution continues?

Is there something equivalent to

WaitHandle.WaitAll(...);

going on?

So If I call something like

Parallel.ForEach(collection, DoSomethingWithObject);

Can I be certain that each call to DoSomethingWithObject has completed before ForEach returns, or do I need to use WaitHandles myself?

Answer 1

All the tasks will have completed by the time Parallel.ForEach() returns, unless a call to Stop() was specifically made to abort the parallel execution. In such cases, the .IsCompleted property of the ParallelLoopState object returned by Parallel.ForEach() will be false.

You can read about it here:

http://msdn.microsoft.com/en-us/library/dd992001.aspx

http://msdn.microsoft.com/en-us/library/system.threading.tasks.parallelloopresult.aspx

You can also write a small test application where DoSomethingWithObject() is a very long running operation, or a call to Thread.Sleep(), to verify this. Execution will block until all threads have finished sleeping.

Sample:

Parallel.ForEach(Enumerable.Range(1, 8),
    i =>
    {
        Thread.Sleep(5000);
        Console.WriteLine(i + " done!");
    });
Console.WriteLine("All done!");