报错Argument 2: cannot convert from 'System.Threading.Tasks.ParallelLoopState' to 'System.Threading.CancellationToken' 如何解决？

Question

Following on:

How to download files using HttpClient with a ProgressBar?

The project is WinForms .NET 6

How to solve the erro:

Severity Code Description Project File Line Suppression State Error CS1503 Argument 2: cannot convert from 'System.Threading.Tasks.ParallelLoopState' to 'System.Threading.CancellationToken' WinFormsApp1 D:\Csharp\WinFormsApp1\ResourceDownloader.cs 63 Active

On line number 63

var dataBytes = await client.Value.GetByteArrayAsync(site, token);

The full class code:

using System;
using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Net;
using System.Text;
using System.Threading.Tasks;

namespace WinFormsApp1
{
    public class ResourceDownloader
    {
        private static Lazy<HttpClient> client = new(() => {
            HttpClientHandler handler = CreateHandler(autoRedirect: true);

            var client = new HttpClient(handler, true) { Timeout = TimeSpan.FromSeconds(60) };
            client.DefaultRequestHeaders.Add("User-Agent", @"Mozilla/5.0 (Windows NT 10; Win64; x64; rv:56.0) Gecko/20100101 Firefox/56.0");
            client.DefaultRequestHeaders.Add("Cache-Control", "no-cache");
            client.DefaultRequestHeaders.Add("Accept-Encoding", "gzip, deflate");
            client.DefaultRequestHeaders.ConnectionClose = true;
            return client;
        }, true);

        private static HttpClientHandler CreateHandler(bool autoRedirect)
        {
            return new HttpClientHandler()
            {
                AllowAutoRedirect = autoRedirect,
                CookieContainer = new CookieContainer(),
                AutomaticDecompression = DecompressionMethods.GZip | DecompressionMethods.Deflate
            };
        }

        public record Website(string Url, byte[]? Data, bool Completed = true, Exception? Ex = null);
        public record ProgressReport(Website Site, int PercentageComplete);

        private static object syncObj = new object();
        private static ConcurrentBag<Website> processed = default!;
        private static int progressCount = 0;
        private static int totalCount = 0;

        public static bool IsBusy { get; internal set; } = false;

        public static async Task<List<Website>> Download(IProgress<ProgressReport> progress, IList<string> sites, CancellationTokenSource cts)
        {
            IsBusy = true;
            processed = new ConcurrentBag<Website>();
            progressCount = 0;
            totalCount = sites.Count;

            try
            {
                ParallelOptions options = new()
                {
                    MaxDegreeOfParallelism = 8,
                    CancellationToken = cts.Token
                };

                await Parallel.ForEach(sites, options, async (site, token) => {
                    try
                    {
                        var dataBytes = await client.Value.GetByteArrayAsync(site, token);
                        ReportProgress(progress, dataBytes, site, null);
                    }
                    catch (Exception ex)
                    {
                        ReportProgress(progress, null, site, ex);
                    }
                });
            }
            // To Debug / Log
            catch (TaskCanceledException) { Debug.Print("The operation was canceled"); }
            finally { IsBusy = false; }
            return processed.ToList();
        }

        private static void ReportProgress(IProgress<ProgressReport> progress, byte[]? data, string site, Exception? ex)
        {
            lock (syncObj)
            {
                progressCount += 1;
                var percentage = progressCount * 100 / totalCount;
                Website website = new(site, data, ex is null, ex);
                processed.Add(website);
                progress.Report(new ProgressReport(website, percentage));
            }
        }
    }
}

Answer 1

Parallel.ForEach is not Task -aware and does not handle asynchronous workloads (correctly, at least), and overloads accepting handler with 2 parameters (like ForEach<TSource>(IEnumerable<TSource>, Action<TSource,ParallelLoopState>) ) use ParallelLoopState as second parameter of the handler.

You need to use Parallel.ForEachAsync (for example this overload , which accepts Func<TSource, CancellationToken, ValueTask> ) to correctly handle async workloads.