Downloader类别：

using System;
using System.Buffers;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;

namespace Download_Http
{
    class Downloader
    {
        public delegate void DownloadProgressHandler(long? totalFileSize, long totalBytesDownloaded, double? progressPercentage);

        public static class DownloadWithProgress
        {
            public static async Task ExecuteAsync(HttpClient httpClient, string downloadPath, string destinationPath, DownloadProgressHandler progress, Func<HttpRequestMessage> requestMessageBuilder = null)
            {
                if (requestMessageBuilder != null)
                    GetDefaultRequestBuilder(downloadPath);
                var download = new HttpClientDownloadWithProgress(httpClient, destinationPath, requestMessageBuilder);
                download.ProgressChanged += progress;
                await download.StartDownload();
                download.ProgressChanged -= progress;
            }

            private static Func<HttpRequestMessage> GetDefaultRequestBuilder(string downloadPath)
            {
                return () => new HttpRequestMessage(HttpMethod.Get, downloadPath);
            }
        }

        internal class HttpClientDownloadWithProgress
        {
            private readonly HttpClient _httpClient;
            private readonly string _destinationFilePath;
            private readonly Func<HttpRequestMessage> _requestMessageBuilder;
            private int _bufferSize = 8192;

            public event DownloadProgressHandler ProgressChanged;

            public HttpClientDownloadWithProgress(HttpClient httpClient, string destinationFilePath, Func<HttpRequestMessage> requestMessageBuilder)
            {
                _httpClient = httpClient ?? throw new ArgumentNullException(nameof(httpClient));
                _destinationFilePath = destinationFilePath ?? throw new ArgumentNullException(nameof(destinationFilePath));
                _requestMessageBuilder = requestMessageBuilder ?? throw new ArgumentNullException(nameof(requestMessageBuilder));
            }

            public async Task StartDownload()
            {
                var requestMessage = _requestMessageBuilder.Invoke();
                var response = await _httpClient.SendAsync(requestMessage, HttpCompletionOption.ResponseHeadersRead);
                await DownloadAsync(response);
            }

            private async Task DownloadAsync(HttpResponseMessage response)
            {
                response.EnsureSuccessStatusCode();

                var totalBytes = response.Content.Headers.ContentLength;

                using (var contentStream = await response.Content.ReadAsStreamAsync())
                    await ProcessContentStream(totalBytes, contentStream);
            }

            private async Task ProcessContentStream(long? totalDownloadSize, Stream contentStream)
            {
                var totalBytesRead = 0L;
                var readCount = 0L;
                var buffer = ArrayPool<byte>.Shared.Rent(_bufferSize);
                var isMoreToRead = true;

                using (var fileStream = new FileStream(_destinationFilePath, FileMode.Create, FileAccess.Write, FileShare.None, _bufferSize, true))
                {
                    do
                    {
                        var bytesRead = await contentStream.ReadAsync(buffer, 0, buffer.Length);
                        if (bytesRead == 0)
                        {
                            isMoreToRead = false;
                            ReportProgress(totalDownloadSize, totalBytesRead);
                            continue;
                        }

                        await fileStream.WriteAsync(buffer, 0, bytesRead);

                        totalBytesRead += bytesRead;
                        readCount += 1;

                        if (readCount % 100 == 0)
                            ReportProgress(totalDownloadSize, totalBytesRead);
                    }
                    while (isMoreToRead);
                }

                ArrayPool<byte>.Shared.Return(buffer);
            }

            private void ReportProgress(long? totalDownloadSize, long totalBytesRead)
            {
                double? progressPercentage = null;
                if (totalDownloadSize.HasValue)
                    progressPercentage = Math.Round((double)totalBytesRead / totalDownloadSize.Value * 100, 2);

                ProgressChanged?.Invoke(totalDownloadSize, totalBytesRead, progressPercentage);
            }
        }
    }
}

使用它的形式1：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Download_Http
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private async void button1_Click(object sender, EventArgs e)
        {
            HttpClient client = new HttpClient();
            const string url = "https://speed.hetzner.de/100MB.bin";

            Downloader.DownloadProgressHandler progressHandler = null;
            await Downloader.DownloadWithProgress.ExecuteAsync(client,url, @"D:\Test\100MB.bin", progressHandler, () =>
            {
                var requestMessage = new HttpRequestMessage(HttpMethod.Get, url);
                requestMessage.Headers.Accept.TryParseAdd("application/octet-stream");
                return requestMessage;
            });
        }
    }
}

它正在下载文件，但我如何向progressBar1报告进度？我在form1设计器中有一个ProgressBar控件。我如何向标签报告下载速度和其他计算结果？
如何创建进度更改事件和完成事件？