Parallel.ForEach and async-await

Solution 1:

async doesn't work well with ForEach. In particular, your async lambda is being converted to an async void method. There are a number of reasons to avoid async void (as I describe in an MSDN article); one of them is that you can't easily detect when the async lambda has completed. ASP.NET will see your code return without completing the async 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:

public async Task<MyResult> GetResult()
{
  MyResult result = new MyResult();

  var tasks = Methods.Select(method => ProcessAsync(method)).ToArray();
  string[] json = await Task.WhenAll(tasks);

  result.Prop1 = PopulateProp1(json[0]);
  ...

  return result;
}

Solution 2:

Alternatively, with the AsyncEnumerator NuGet Package you can do this:

using System.Collections.Async;

public async Task<MyResult> GetResult()
{
    MyResult result = new MyResult();

    await Methods.ParallelForEachAsync(async method =>
    {
        string json = await Process(method);    

        result.Prop1 = PopulateProp1(json);
        result.Prop2 = PopulateProp2(json);
    }, maxDegreeOfParallelism: 10);

    return result;
}

where ParallelForEachAsync is an extension method.