Is it OK to do some async/await inside some .NET Parallel.ForEach() code?

Question

Given the following code, is it OK to do async/await inside a Parallel.ForEach ?

eg.

Parallel.ForEach(names, name          


        

Answer 1

No, It doesn't make sense to combine async with Paralell.Foreach.

Consider the following example:

private void DoSomething()
{
    var names = Enumerable.Range(0,10).Select(x=> "Somename" + x);
    Parallel.ForEach(names, async(name) =>
    {   
        await Task.Delay(1000);
        Console.WriteLine("Name {0} completed",name);
    });
    Console.WriteLine("Parallel ForEach completed");
}

What output you will expect?

Name Somename3 completed
Name Somename8 completed
Name Somename4 completed
...
Parallel ForEach completed

That's not what will happen. It will output :

Parallel ForEach completed
Name Somename3 completed
Name Somename8 completed
Name Somename4 completed
...

Why? Because when ForEach hits first await the method actually returns, Parallel.ForEach doesn't know it is asynchronous and it ran to completion!. Code after await runs as continuation on another thread not "Paralell processing thread"

Stephen toub addressed this here