c# - 并行下载大量文件的有效方法

Question

我正在尝试从互联网下载大量文件(图片)。我正在纠结异步/并行，因为
a) 我不能说是否有文件。我刚收到一百万个链接，其中包含一张图片(300kb 到 3MB)或 404 页面不存在。因此，为了避免下载 0 字节文件，我两次询问同一页面，一次是 404，然后是图片。另一种方法是下载所有 0 字节文件并在之后删除数百万个文件 - 这使 Windows 10 一直停留在此任务上，直到我重新启动。
b) 虽然(非常慢的)下载正在进行中，但每当我查看任何“成功下载的文件”时，它都是用 0 字节创建的，并且不包含图片。我需要更改什么才能在下载下一个文件之前真正下载文件？
我如何解决这两个问题？有没有更好的方法来下载数以百万计的文件(在服务器上压缩/创建 .zip 是不可能的)

           //loopResult = Parallel.ForEach(_downloadLinkList, new ParallelOptions { MaxDegreeOfParallelism = 10 }, DownloadFilesParallel);    
            private async void DownloadFilesParallel(string path)
            {
                string downloadToDirectory = ""; 
                string x = ""; //in case x fails, i get 404 from webserver and therefore no download is needed
                System.Threading.Interlocked.Increment(ref downloadCount);
                OnNewListEntry(downloadCount.ToString() + " / " + linkCount.ToString() + " heruntergeladen"); //tell my gui to update
                try
                {
                    using(WebClient webClient = new WebClient())
                    {
                        downloadToDirectory = Path.Combine(savePathLocalComputer, Path.GetFileName(path)); //path on local computer

                        webClient.Credentials = CredentialCache.DefaultNetworkCredentials;
                        x = await webClient.DownloadStringTaskAsync(new Uri(path)); //if this throws an exception, ignore this link
                        Directory.CreateDirectory(Path.GetDirectoryName(downloadToDirectory)); //if request is successfull, create -if needed- the folder on local pc
                        await webClient.DownloadFileTaskAsync(new Uri(path), @downloadToDirectory); //should download the file, release 1 parallel task to get the next file. instead there is a 0-byte file and the next one will be downloaded
                    }
                }
                catch(WebException wex)
                {
                }
                catch(Exception ex)
                {
                    System.Diagnostics.Debug.WriteLine(ex.Message);
                }
                finally
                {
                    
                }
            }

//图片是sfw，链接是nsfw

Answer 1

这是使用 HttpClient 的示例具有最大并发下载限制。

private static readonly HttpClient client = new HttpClient();

private async Task DownloadAndSaveFileAsync(string path, SemaphoreSlim semaphore, IProgress<int> status)
{
    try
    {
        status?.Report(semaphore.CurrentCount);
        using (HttpResponseMessage response = await client.GetAsync(path, HttpCompletionOption.ResponseHeadersRead).ConfigureAwait(false))
        {
            if (response.IsSuccessStatusCode) // ignoring if not success
            {
                string filePath = Path.Combine(savePathLocalComputer, Path.GetFileName(path));
                string dir = Path.GetDirectoryName(filePath);
                if (!Directory.Exists(dir)) Directory.CreateDirectory(dir);
                using (Stream responseStream = await response.Content.ReadAsStreamAsync().ConfigureAwait(false))
                using (FileStream fileStream = File.Create(filePath))
                {
                    await responseStream.CopyToAsync(fileStream).ConfigureAwait(false);
                }
            }
        }
    }
    finally
    {
        semaphore.Release();
    }
}

并发性

client.BaseAddress = "http://somesite";
int downloadCount = 0;
List<string> pathList = new List<string>();
// fill the list here

List<Task> tasks = new List<Task>();
int maxConcurrentTasks = Environment.ProcessorCount * 2; // 16 for me

IProgress<int> status = new Progress<int>(availableTasks =>
{
    downloadCount++;
    OnNewListEntry(downloadCount + " / " + pathList.Count + " heruntergeladen\r\nRunning " + (maxConcurrentTasks - availableTasks) + " downloads.");
});

using (SemaphoreSlim semaphore = new SemaphoreSlim(maxConcurrentTasks))
{
    foreach (string path in pathList)
    {
        await semaphore.WaitAsync();
        tasks.Add(DownloadAndSaveFileAsync(path, semaphore, status));
    }
    try
    {
        await Task.WhenAll(tasks);
    }
    catch (Exception ex)
    {
        // handle the Exception here
    }
}

Progress这里只是在 UI 线程上执行回调。因此 Interlocked内部不需要，更新 UI 是安全的。
如果使用 .NET Framework(在 .NET Core 中没有效果但不需要)以使其更快，您可以将此行添加到应用程序启动代码中

ServicePointManager.DefaultConnectionLimit = 10;