Combining PLINQ with Async method

Question

I'm trying to combine my PLINQ statement like this:

Enumerable.Range(0, _sortedList.Count()).AsParallel().WithDegreeOfParallelism(10)
          .Select(i =>  GetTransactionDetails(_sortedList[i].TransactionID))
          .ToList();

With an async method like this:

 private async void GetTransactionDetails(string _trID)
 {
      await Task.Run(() =>
      {
      });
 }

So that I can simply add an await operator in here:

 Enumerable.Range(0, _sortedList.Count()).AsParallel().WithDegreeOfParallelism(10)
           .Select(i => await GetTransactionDetails(_sortedList[i].TransactionID))
           .ToList();

How can I achieve this ?

P.S. This way I could make 5-10 HTTP requests simultaneously while ensuring that the end user doesn't feels any "screen" freezing while doing so...

Answer 1

There's a couple approaches you can take.

First, the "more correct" approach (which is also more work). Make GetTransactionDetails into a proper async method (i.e., does not use Task.Run):

private async Task GetTransactionDetailsAsync(string _trID);

Then you can call that method concurrently:

var tasks = _sortedList.Select(x => GetTransactionDetailsAsync(x.TransactionID));
await Task.WhenAll(tasks);

If you need to limit concurrency, use a SemaphoreSlim.

The second approach is more wasteful (in terms of thread usage), but is probably easier given what parts of your code we've seen. The second approach is to leave the I/O all synchronous and just do it in a regular PLINQ fashion:

private void GetTransactionDetails(string _trID);

_sortedList.AsParallel().WithDegreeOfParallelism(10).Select(x => GetTransactionDetails(x.TransactionID)).ToList();

To avoid blocking the UI thread, you can wrap this in a single Task.Run:

await Task.Run(() => _sortedList.AsParallel().WithDegreeOfParallelism(10).Select(x => GetTransactionDetails(x.TransactionID)).ToList());