I had such method:
public async Task<MyResult> GetResult()
{
MyResult result = new MyResult();
foreach(var method in Methods)
{
string json = await Process(method);
result.Prop1 = PopulateProp1(json);
result.Prop2 = PopulateProp2(json);
}
return result;
}
Then I decided to use Parallel.ForEach
:
public async Task<MyResult> GetResult()
{
MyResult result = new MyResult();
Parallel.ForEach(Methods, async method =>
{
string json = await Process(method);
result.Prop1 = PopulateProp1(json);
result.Prop2 = PopulateProp2(json);
});
return result;
}
But now I've got an error:
An asynchronous module or handler completed while an asynchronous operation was still pending.
async
doesn't work well withForEach
. In particular, yourasync
lambda is being converted to anasync void
method. There are a number of reasons to avoidasync void
(as I describe in an MSDN article); one of them is that you can't easily detect when theasync
lambda has completed. ASP.NET will see your code return without completing theasync void
method and (appropriately) throw an exception.What you probably want to do is process the data concurrently, just not in parallel. Parallel code should almost never be used on ASP.NET. Here's what the code would look like with asynchronous concurrent processing:
Alternatively, with the AsyncEnumerator NuGet Package you can do this:
where
ParallelForEachAsync
is an extension method.Ahh, okay. I think I know what's going on now.
async method =>
an "async void" which is "fire and forget" (not recommended for anything other than event handlers). This means the caller cannot know when it is completed... So,GetResult
returns while the operation is still running. Although the technical details of my first answer are incorrect, the result is the same here: that GetResult is returning while the operations started byForEach
are still running. The only thing you could really do is notawait
onProcess
(so that the lambda is no longerasync
) and wait forProcess
to complete each iteration. But, that will use at least one thread pool thread to do that and thus stress the pool slightly--likely making use ofForEach
pointless. I would simply not use Parallel.ForEach...