c# - 如果失败，如何让webclient重新下载文件？

Question

我正在尝试使用 foreach 将图像链接列表下载到我的服务器(最多 40 个链接)。

在我的情况下，有时链接存在，但我不知道为什么它会捕获并取消下一个链接的下载。也许需要稍等一下？因为当我调试应用程序时，我看到链接是应用程序跳过并去捕获可用但有时它在我的浏览器中几秒钟后打开所以我尝试下载的服务器的响应时间有时需要更多时间来加载和打开链接。

   string newPath = "~/data/" + model.PostID + "/" + name + "/";

   //test1 is a list of links
                foreach (var item1 in test1)
                {

                    HttpWebRequest request = WebRequest.Create(item1) as HttpWebRequest; request.Method = "HEAD";
                    try
                    {
                        using (HttpWebResponse response = request.GetResponse() as HttpWebResponse)
                        {
                            var webClient = new WebClient();
                            string path = newPath + i + ".jpg";
                            webClient.DownloadFileAsync(new Uri(item1), Server.MapPath(path));
                            string newlinks = "https://example.com/data/" + chapter.PostID + "/" + name + "/" + i + ".jpg";
                            allimages = allimages + newlinks + ',';
                            response.Close();
                            i++;

                        }
                    }

                    catch
                    {                           
                        break;
                    }



                }

现在我的主要目标是解决这个问题，但正如我在调试中看到的那样:

1. 我要下载的图片链接存在

2. 有时需要更多时间来响应

那么我该如何解决这个问题？当下载取消并且存在链接时，我应该怎么做？

Answer 1

你可以使用这个例子:

    class WebClientUtility : WebClient
    {
        public int Timeout { get; set; }

        public WebClientUtility() : this(60000) { }

        public WebClientUtility(int timeout)
        {
            this.Timeout = timeout;
        }

        protected override WebRequest GetWebRequest(Uri address)
        {
            var request = base.GetWebRequest(address);
            if (request != null)
            {
                request.Timeout = Timeout;
            }
            return request;
        }
    }

    //
    public class DownloadHelper : IDisposable
    {
        private WebClientUtility _webClient;
        private string _downloadUrl;
        private string _savePath;
        private int _retryCount;

        public DownloadHelper(string downloadUrl, string savePath)
        {
            _savePath = savePath;
            _downloadUrl = downloadUrl;

            _webClient = new WebClientUtility();

            _webClient.DownloadFileCompleted += ClientOnDownloadFileCompleted;
        }


        public void StartDownload()
        {
            _webClient.DownloadFileAsync(new Uri(_downloadUrl), _savePath);
        }

        private void ClientOnDownloadFileCompleted(object sender, AsyncCompletedEventArgs e)
        {
            if (e.Error != null)
            {
                _retryCount++;

                if (_retryCount < 3)
                {
                    _webClient.DownloadFileAsync(new Uri(_downloadUrl), _savePath);
                }
                else
                {
                    Console.WriteLine(e.Error.Message);
                }
            }
            else
            {
                _retryCount = 0;
                Console.WriteLine($"successfully download: # {_downloadUrl}  to  # {_savePath}");
            }
        }
        public void Dispose()
        {
            _webClient.Dispose();
        }
    }

    //
    class Program
    {

        private static void Main(string[] args)
        {
            for (int i = 0; i < 100; i++)
            {
                var downloadUrl = $@"https://example.com/mag-{i}.pdf";
                var savePath = $@"D:\DownloadFile\FileName{i}.pdf";

                DownloadHelper downloadHelper = new DownloadHelper(downloadUrl, savePath);

                downloadHelper.StartDownload();
            }

            Console.ReadLine();
        }
    }

要解决超时问题，您可以创建一个派生类并设置 WebRequest 基类的超时属性，然后对于重试，您可以使用 WebClient 的 DownloadFileCompleted 事件并在那里实现您的重试模式