Download with Httpwebclient only works outside of Parallel.Foreach

Question

I am developing a class for downloading C files#: Filedownloader. The problem is occurring in using it. I am trying to make simultaneous downloads, it is working normal when it is running outside the Parallel but when it spins inside it using (var response = await request.GetResponseAsync()) // line 205 da classe DownloadFile is waiting forever ignoring the TimeOut and does not download.

    private async void DoDownload(string[] segments)
    {                   
        //Whether to stop downloading
        stopdownload = false;
        cts = new CancellationTokenSource();

        //Multi-threaded setup
        ParallelOptions parallelOptions = new ParallelOptions
        {
            MaxDegreeOfParallelism = 2,
            CancellationToken = cts.Token
        };

        //Start Downloading
        OnDownloadStart?.Invoke(this, EventArgs.Empty);
        DownloadStatus = DownloadStatus.Downloading;


        var downloader1 = new Downloader.DownloadFile()
        {
            Url = @"https://github.com/BtbN/FFmpeg-Builds/releases/download/autobuild-2021-03-13-12-31/ffmpeg-n4.3.2-160-gfbb9368226-win64-gpl-4.3.zip",
            FilePath = Path.Combine(OutFolder.FullName, $"part_10.zip")
        };
        await downloader1.StartAsync();


        try
        {
            Parallel.ForEach(segments, parallelOptions,  async (info, loopstate, index) =>
                {
                    if (stopdownload)
                        loopstate.Stop();

                    var downloader2 = new Downloader.DownloadFile()
                    {
                        Url = @"https://github.com/BtbN/FFmpeg-Builds/releases/download/autobuild-2021-03-13-12-31/ffmpeg-n4.3.2-160-gfbb9368226-win64-gpl-4.3.zip",
                        FilePath = Path.Combine(OutFolder.FullName, $"part_{index}.zip")
                    };
                    await downloader2.StartAsync();

                });
        }
        catch (OperationCanceledException)
        {
            ;
        }
    }

The downloader1 drops normally while the downloader2 no. I fixed the url to one that I was sure was working. But why is this happening?

Answer 1

The objective of Parallel.ForEach is to parallelize Tasks but not with async/await, where you hope to return a Task and hope that it is completed.

The Parallel.ForEach shall work well with another synchronous method of WebClient.

To make async, has a much simpler approach that is to use own System.Linq to create Tasks and await their outcome, something like this:

// Para cada "segment" crio uma Task de download
var downloadsTasks = segments.Select(segment =>
{
    return new Downloader.DownloadFile
    {
          Url = segment,
          FilePath = Path.Combine(OutFolder.FullName, $"part_{segment}.zip")
    }.StartAsync();    
});

// Aguardo todos os downloads terminarem.
var downloads = await Task.WhenAll(downloadsTasks);

Assuming "Segments" has the download links or can mount the download name within the .Select. This ensures you will return Tasks and will do the await next.