WebClient.DownloadFileAsync - 一次下载一个文件 [英] WebClient.DownloadFileAsync - Download files one at a time

问题描述

我正在使用以下代码从 TFS 服务器下载多个附件:

I am using the code below to download multiple attachments from a TFS server:

foreach (Attachment a in wi.Attachments)
{    
    WebClient wc = new WebClient();
    wc.Credentials = (ICredentials)netCred;
    wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileCompleted);
    wc.DownloadFileAsync(a.Uri, "C:\" + a.Name);
}

我想使用 DownloadFileAsync 下载多个文件，但我希望它们一个一个地下载.

I would like to download multiple files using DownloadFileAsync, but I want them to be downloaded one by one.

有人可能会问为什么不直接使用同步 DownloadFile 方法?"因为:

One may ask "Why don't you just use the synchronous DownloadFile method?" Its because:

1. 我想利用 DownloadFileAsync 提供的事件.
2. 我不想创建多个 Web 客户端实例以避免淹没服务器.

这是我想到的解决方案:

This is the solution that I thought of:

foreach (Attachment a in wi.Attachments)
{        
    WebClient wc = new WebClient();
    wc.Credentials = (ICredentials)netCred;
    wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileCompleted);
    wc.DownloadFileAsync(a.Uri, "C:\" + a.Name);
    while (wc.IsBusy)
    {
        System.Threading.Thread.Sleep(1000);
    }
}

但是，这种方法存在一些问题:

However, there are a couple of problems with this approach:

1. Thread.Sleep() 正在锁定我的表单.我仍然需要创建自己的线程或使用BackgroundWorker.(我想尽可能避免这种情况)
2. DownloadFileCompleted 事件在 ALL 文件下载后被触发.我不知道这是否是使用 System.Threading.Thread.Sleep(1000) 的副作用；

1. The Thread.Sleep() is locking up my Form. I still need to make my own Thread or use BackgroundWorker. (I would like to avoid this as much as possible)
2. The DownloadFileCompleted event is being triggered after ALL files has been downloaded. I don't know if this is a side-effect of using System.Threading.Thread.Sleep(1000);

是否有更好的方法使用 WebClient.DownloadFileAsync 一次下载一个文件?

Is there a better approach to download files one at a time using WebClient.DownloadFileAsync?

谢谢！

推荐答案

为了简化任务，您可以创建单独的附件列表:

To simplify the task you can create separated attachment list:

list = new List<Attachment>(wi.Attachments);

其中 list 是类型为 List 的私有字段.在此之后，您应该配置 WebClient 并开始下载第一个文件:

where list is private field with type List<Attachment>. After this you should configure WebClient and start downloading of first file:

if (list.Count > 0) {
   WebClient wc = new WebClient();
   wc.Credentials = (ICredentials)netCred;
   wc.DownloadFileCompleted += new AsyncCompletedEventHandler(wc_DownloadFileCompleted);
   wc.DownloadFileAsync(list[0].Uri, @"C:" + list[0].Name);
}

您的 DownloadFileComplete 处理程序应检查是否所有文件都已下载并再次调用 DownloadFileAsync:

Your DownloadFileComplete handler should check if not all files already downloaded and call DownloadFileAsync again:

void wc_DownloadFileCompleted(object sender, AsyncCompletedEventArgs e) {
   // ... do something useful 
   list.RemoveAt(0);
   if (list.Count > 0)
      wc.DownloadFileAsync(list[0].Uri, @"C:" + list[0].Name);
}

此代码不是优化的解决方案.这只是想法.

This code is not optimized solution. This is just idea.

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