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Question

EvgeniyZ

Asked:2020-08-11 05:22:45 +0000 UTC2020-08-11 05:22:45 +0000 UTC 2020-08-11 05:22:45 +0000 UTC

WebClient 文件下载部分的大小

772

有一项任务是从带有进度显示（总和当前文件大小）的 Web 服务器下载约 3000 个文件。也就是为了方便用户，需要实现文件的总大小和总共下载多少（例如：100mb out of 300mb download）。

我从服务器以 JSON 数据的形式获取上传文件的总大小（它提供了一个列表，其中包含有关每个文件、其大小、名称等的信息），但当前大小存在困难......

我使用WebClinet，我从中捕获事件DownloadProgressChanged。在这个事件里面，我们可以得到BytesReceived，也就是我们已经从这个文件下载了多少的大小，它一直在增长，直到它变成 = 文件的完整大小。

我的任务是计算在一次事件调用中下载了多少字节DownloadProgressChanged，而不是总数。我试图将前一个保存在一个BytesReceived单独的变量中（ 11561291193（也就是差不多一半，但是所有文件都接收成功！计数不正确...）。oldSizevar size = e.BytesReceived - oldSize;

请帮助我实现从服务器下载的字节的有效输出......

当前实施：

private void RepairClient()
{
    var filesBlock = new BufferBlock<FileViewModel>();
    var generateOptions = new ExecutionDataflowBlockOptions
    {
        MaxDegreeOfParallelism = 4,
        BoundedCapacity = 100
    };

    var downloadBlock = new ActionBlock<FileViewModel>(async file =>
    {
        SetFileName(file.Name);
        var packPach = file.Name + ServerData.PackName;
        var uri = new Uri(main.UpdaterViewModel.Path, packPach);
        var fullPackPach = Model.StorageModel.ExpandRelativeName(packPach);
        await file.DownloadFile(uri, fullPackPach, main);
    }, generateOptions);


    var linkOptions = new DataflowLinkOptions { PropagateCompletion = true };
    filesBlock.LinkTo(downloadBlock, linkOptions);

    var totalSize = UpdateFiles.Sum(x => x.FileSize);

    SetMinMax(0, UpdateFiles.Count);
    SetSize(total: totalSize); //Вывести в интерфейс полный размер.
    SetMinMax(0, totalSize); //Работа с ProgressBar (задать min и max).
    SetStatus(true);

    UpdateFiles.ForEach(x => filesBlock.Post(x));
    filesBlock.Complete();
    downloadBlock.Completion.Wait();
}

我尝试使用简单的方法编写方法acync，现在我正在玩Dataflow，但无济于事。在 blockdownloadBlock中，我们做通常的准备（我们计算路径）并调用DownloadFile当前文件的方法。

方法DownloadFile：

public async Task DownloadFile(Uri uri, string path, MainViewModel main)
{
    try
    {
        using (var client = new WebClient())
        {
            long prev = 0;
            client.DownloadProgressChanged += (sender, args) =>
            {
                var diff = args.BytesReceived - prev;
                main.Progress.Size += diff;
                prev = args.BytesReceived;

                Console.WriteLine($"[{path}]: BytesReceived: {args.BytesReceived} (diff: {diff})");
            };

            var dir = Path.GetDirectoryName(path);
            if (dir != null && !Directory.Exists(dir)) Directory.CreateDirectory(dir);

            await client.DownloadFileTaskAsync(uri, path);
        }
    }
    catch (Exception e)
    {
        Console.WriteLine(e);
    }
}

这是结果的输出：

[patchw32.dll.client]: BytesReceived: 10215 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 3370 (diff: -2738)
[patchw32.dll.client]: BytesReceived: 11584 (diff: 9583)
[patchw32.dll.client]: BytesReceived: 2001 (diff: 2001)
[patchw32.dll.client]: BytesReceived: 6108 (diff: 6108)
[patchw32.dll.client]: BytesReceived: 8846 (diff: 8846)
[patchw32.dll.client]: BytesReceived: 4739 (diff: -6845)
[patchw32.dll.client]: BytesReceived: 12953 (diff: 8214)
[patchw32.dll.client]: BytesReceived: 7477 (diff: -5476)
[patchw32.dll.client]: BytesReceived: 14322 (diff: 6845)
[patchw32.dll.client]: BytesReceived: 15691 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 17060 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 18429 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 19798 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 21167 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 22536 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 23905 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 25274 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 26643 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 28012 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 29381 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 30750 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 32031 (diff: 1281)
[patchw32.dll.client]: BytesReceived: 33400 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 34769 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 36138 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 37507 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 38876 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 40245 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 41614 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 42983 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 44352 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 45721 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 47090 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 48459 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 49828 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 51197 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 52566 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 53935 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 55304 (diff: 1369)
[patchw32.dll.client]: BytesReceived: 59533 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 63762 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 64799 (diff: 1037)
[patchw32.dll.client]: BytesReceived: 69028 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 73257 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 77486 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 81715 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 85944 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 90173 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 94402 (diff: 4229)
[patchw32.dll.client]: BytesReceived: 95819 (diff: 1417)
[patchw32.dll.client]: BytesReceived: 95819 (diff: 0)
[L2text\help_battle_shot.htm.client]: BytesReceived: 1067 (diff: 1067)
[L2text\help_battle_shot.htm.client]: BytesReceived: 1067 (diff: 1067)
[L2text\help_item_symbol.htm.client]: BytesReceived: 936 (diff: 936)
[L2text\help_item_symbol.htm.client]: BytesReceived: 936 (diff: 936)
[L2text\help_option_video.htm.client]: BytesReceived: 2337 (diff: 2337)
[L2text\help_option_video.htm.client]: BytesReceived: 2337 (diff: 2337)
[L2text\credit_us_03.htm.client]: BytesReceived: 907 (diff: 907)
[L2text\credit_us_03.htm.client]: BytesReceived: 907 (diff: 907)
[L2text\pet_help_quest_2.htm.client]: BytesReceived: 809 (diff: 809)
[L2text\pet_help_quest_2.htm.client]: BytesReceived: 809 (diff: 809)
[music\S09_F.ogg.client]: BytesReceived: 1992 (diff: 1992)
[music\S09_F.ogg.client]: BytesReceived: 3361 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 4730 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 6099 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 10206 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 11575 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 7468 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 8837 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 14313 (diff: 2738)
[music\S09_F.ogg.client]: BytesReceived: 12944 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 15682 (diff: 2738)
[music\S09_F.ogg.client]: BytesReceived: 17051 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 18420 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 19789 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 21158 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 22527 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 23896 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 25265 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 26634 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 28003 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 29372 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 30741 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 31384 (diff: 643)
[music\S09_F.ogg.client]: BytesReceived: 32753 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 34122 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 35491 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 36860 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 38229 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 39598 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 40967 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 42336 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 43705 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 45074 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 46443 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 47812 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 49181 (diff: 1369)
[music\S09_F.ogg.client]: BytesReceived: 53410 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 57639 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 61868 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 64152 (diff: 2284)
[music\S09_F.ogg.client]: BytesReceived: 68381 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 72610 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 76839 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 81068 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 85297 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 89526 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 93755 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 96920 (diff: 3165)
[music\S09_F.ogg.client]: BytesReceived: 101149 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 105378 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 109607 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 113836 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 118065 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 122294 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 126523 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 129688 (diff: 3165)
[music\S09_F.ogg.client]: BytesReceived: 133917 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 138146 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 142375 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 146604 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 150833 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 155062 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 159291 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 162456 (diff: 3165)
[music\S09_F.ogg.client]: BytesReceived: 166685 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 170914 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 175143 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 179372 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 183601 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 187830 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 192059 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 195224 (diff: 3165)
[music\S09_F.ogg.client]: BytesReceived: 199453 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 203682 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 207911 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 212140 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 227992 (diff: 15852)
[music\S09_F.ogg.client]: BytesReceived: 244376 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 260760 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 277144 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 293528 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 309912 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 326296 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 342680 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 359064 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 375448 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 3483 (diff: 3483)
[music\sc08_f.ogg.client]: BytesReceived: 7712 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 11941 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 16170 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 20399 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 24628 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 28857 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 31383 (diff: 2526)
[music\S09_F.ogg.client]: BytesReceived: 391832 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 35612 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 39841 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 44070 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 48299 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 408216 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 52528 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 56757 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 60986 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 64151 (diff: 3165)
[music\sc08_f.ogg.client]: BytesReceived: 68380 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 72609 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 76838 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 81067 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 424600 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 440984 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 85296 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 89525 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 93754 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 96919 (diff: 3165)
[music\sc08_f.ogg.client]: BytesReceived: 101148 (diff: 4229)
[music\sc08_f.ogg.client]: BytesReceived: 105377 (diff: 4229)
[music\S09_F.ogg.client]: BytesReceived: 457368 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 473752 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 121761 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 129687 (diff: 7926)
[music\S09_F.ogg.client]: BytesReceived: 490136 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 146071 (diff: 16384)
[music\S09_F.ogg.client]: BytesReceived: 501222 (diff: 11086)
[music\S09_F.ogg.client]: BytesReceived: 501222 (diff: 0)
[music\sc08_f.ogg.client]: BytesReceived: 162455 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 178839 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 195223 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 211607 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 227991 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 244375 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 260759 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 277143 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 293527 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 309911 (diff: 16384)
[music\sc08_f.ogg.client]: BytesReceived: 313342 (diff: 3431)
[music\sc08_f.ogg.client]: BytesReceived: 313342 (diff: 0)
[music\SSQ05_F.ogg.client]: BytesReceived: 15638 (diff: 15638)
[music\SSQ05_F.ogg.client]: BytesReceived: 32022 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 48406 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 64790 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 81174 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 97558 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 113942 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 130326 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 146710 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 162459 (diff: 15749)
[music\SSQ05_F.ogg.client]: BytesReceived: 178843 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 195227 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 211611 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 227995 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 244379 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 260763 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 277147 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 293531 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 309915 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 326299 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 342683 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 359067 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 375451 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 391835 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 408219 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 424603 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 440987 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 457371 (diff: 16384)
[music\SSQ05_F.ogg.client]: BytesReceived: 469073 (diff: 11702)
[music\SSQ05_F.ogg.client]: BytesReceived: 469073 (diff: 11702)
[StaticMeshes\Aden_castle_door_S.usx.client]: BytesReceived: 3614 (diff: 3614)
[StaticMeshes\Aden_castle_door_S.usx.client]: BytesReceived: 3614 (diff: 3614)

您可能会注意到 Diff 并不总是达到 0，有时甚至会变为负数……

1 个回答

Voted

tym32167 · Answer 1 · 2020-08-11T07:29:49Z

我假设在事件处理程序中，除了计算（以及重要的是，在计算之前）之外，您还执行一些操作。

例如下面的代码先进行计算（这需要一点时间），然后导致输出到控制台（文件是从网上随机下载的）

using (var client = new WebClient())
{
    var sum = 0L;
    long prev = 0;

    client.DownloadProgressChanged += (sender, args) =>
    {
        var diff = args.BytesReceived - prev;
        sum += diff;
        prev = args.BytesReceived;

        Console.WriteLine($"BytesReceived: {args.BytesReceived} (diff: {diff})");
    };

    await client.DownloadFileTaskAsync(
        new Uri("http://spatialkeydocs.s3.amazonaws.com/FL_insurance_sample.csv.zip"),
        @"D:\temp\sample");

    Console.WriteLine($"SUM :{sum}");
    Console.WriteLine($"Bytes should be: {new FileInfo(@"D:\temp\sample").Length}");
}

结论是正确的：

BytesReceived: 65536 (diff: 65536)
BytesReceived: 118414 (diff: 52878)
BytesReceived: 146814 (diff: 28400)
BytesReceived: 151074 (diff: 4260)
BytesReceived: 216610 (diff: 65536)
BytesReceived: 219234 (diff: 2624)
BytesReceived: 230594 (diff: 11360)
BytesReceived: 270354 (diff: 39760)
BytesReceived: 322894 (diff: 52540)
BytesReceived: 331414 (diff: 8520)
BytesReceived: 334886 (diff: 3472)
BytesReceived: 349874 (diff: 14988)
BytesReceived: 382534 (diff: 32660)
BytesReceived: 386794 (diff: 4260)
BytesReceived: 436494 (diff: 49700)
BytesReceived: 445014 (diff: 8520)
BytesReceived: 448486 (diff: 3472)
BytesReceived: 469154 (diff: 20668)
BytesReceived: 501814 (diff: 32660)
BytesReceived: 514594 (diff: 12780)
BytesReceived: 558614 (diff: 44020)
BytesReceived: 561454 (diff: 2840)
BytesReceived: 621094 (diff: 59640)
BytesReceived: 622514 (diff: 1420)
BytesReceived: 688050 (diff: 65536)
BytesReceived: 711974 (diff: 23924)
BytesReceived: 746054 (diff: 34080)
BytesReceived: 761674 (diff: 15620)
BytesReceived: 804274 (diff: 42600)
BytesReceived: 807114 (diff: 2840)
BytesReceived: 863914 (diff: 56800)
BytesReceived: 866400 (diff: 2486)
BytesReceived: 866400 (diff: 0)
SUM :866400
Bytes should be: 866400

但是，只要我们将输出更改为控制台并在某些地方进行计算（即先放一个慢操作）

using (var client = new WebClient())
{
    var sum = 0L;
    long prev = 0;

    client.DownloadProgressChanged += (sender, args) =>
    {   
        var diff = args.BytesReceived - prev;
        Console.WriteLine($"BytesReceived: {args.BytesReceived} (diff: {diff})");

        sum += diff;
        prev = args.BytesReceived;

        //Console.WriteLine($"BytesReceived: {args.BytesReceived} (diff: {diff})");
    };

    await client.DownloadFileTaskAsync(
        new Uri("http://spatialkeydocs.s3.amazonaws.com/FL_insurance_sample.csv.zip"),
        @"D:\temp\sample");

    Console.WriteLine($"SUM :{sum}");
    Console.WriteLine($"Bytes should be: {new FileInfo(@"D:\temp\sample").Length}");
}

同时，在输出到控制台的过程中，一个并行线程可以在prev被前一个handler更新之前计算一个新的diff（即相同的prev将用于2个diff），这会导致去同步。结论是：

BytesReceived: 16962 (diff: 16962)
BytesReceived: 25482 (diff: 25482)
BytesReceived: 28954 (diff: 3472)
BytesReceived: 45362 (diff: 16408)
BytesReceived: 62402 (diff: 17040)
BytesReceived: 63822 (diff: 1420)
BytesReceived: 83702 (diff: 19880)
BytesReceived: 82282 (diff: 18460)
BytesReceived: 103582 (diff: 21300)
BytesReceived: 104002 (diff: 21720)
BytesReceived: 125302 (diff: 21300)
BytesReceived: 145182 (diff: 19880)
BytesReceived: 146602 (diff: 1420)
BytesReceived: 148022 (diff: 1420)
BytesReceived: 172162 (diff: 24140)
BytesReceived: 173634 (diff: 25612)
BytesReceived: 194934 (diff: 21300)
BytesReceived: 203454 (diff: 29820)
BytesReceived: 221914 (diff: 18460)
BytesReceived: 223334 (diff: 1420)
BytesReceived: 235798 (diff: 12464)
BytesReceived: 265618 (diff: 29820)
BytesReceived: 271298 (diff: 5680)
BytesReceived: 306798 (diff: 35500)
BytesReceived: 309638 (diff: 2840)
BytesReceived: 347978 (diff: 38340)
BytesReceived: 349398 (diff: 1420)
BytesReceived: 390578 (diff: 41180)
BytesReceived: 393418 (diff: 2840)
BytesReceived: 431758 (diff: 38340)
BytesReceived: 439014 (diff: 7256)
BytesReceived: 474514 (diff: 35500)
BytesReceived: 486978 (diff: 12464)
BytesReceived: 533838 (diff: 46860)
BytesReceived: 538098 (diff: 4260)
BytesReceived: 587798 (diff: 49700)
BytesReceived: 592846 (diff: 5048)
BytesReceived: 638286 (diff: 45440)
BytesReceived: 652170 (diff: 13884)
BytesReceived: 707550 (diff: 55380)
BytesReceived: 716070 (diff: 8520)
BytesReceived: 748730 (diff: 32660)
BytesReceived: 762930 (diff: 14200)
BytesReceived: 828250 (diff: 65320)
BytesReceived: 833930 (diff: 71000)
BytesReceived: 866400 (diff: 32470)
BytesReceived: 866400 (diff: 0)
SUM :1035302
Bytes should be: 866400

Отсюда совет - в многопоточном варианте важные и быстрые вычисления помещайте в начало, значения для вывода юзеру запоминайте в локальных переменных (чтобы иъ не могли изменить параллельные потоки). Если это все равно приводит к рассинхрону, то используйте конструкции синхронизации потоков. Например, при потенциальных 2 конкурирующих потоках можно поставить lock

        lock (this)
        {
            var diff = args.BytesReceived - prev;
            Console.WriteLine($"BytesReceived: {args.BytesReceived} (diff: {diff})");

            sum += diff;
            prev = args.BytesReceived;
        }

Но если у вас конкурирующих потоков много, то тогда надо думать основательней. Мне, например, в голову приходят многопоточные очереди, dataflow и прочие вещи. Но это уже тяжелая арта.

И, кстати, я не уверен, что есть вообще какие то гарантии, что эти события идут в хронологическом порядке. Мне кажется поэтому они и не присылают сами дельту, а присылают просто количество скаченных данных.

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