下载多个文件异步并等待所有文件完成后再执行其余代码

Question

我试图从互联网上下载多个文件,等待所有这些文件完成.这是我正在运行的C#控制台应用程序,因此不需要任何进度条事件处理程序.但是,即使尚未下载所有文件,它目前仍继续执行代码.

• 1.下载所有文件!
• 2.完成下载文件A.
• 3.Finished下载所有文件!
• 4.Finished下载文件B.
• 5.Finished下载文件C.

您将如何等待所有异步下载文件完成.

 private void DownloadMultipleFiles(List<DocumentObject> doclist)
    {
        foreach(var value in doclist){
            try
            {
                using (WebClient webClient = new WebClient())
                {
                    string downloadToDirectory = @Resources.defaultDirectory + value.docName;
                    webClient.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;
                    webClient.DownloadFileCompleted += client_DownloadFileCompleted;
                    webClient.DownloadFileAsync(new Uri(value.docUrl), @downloadToDirectory);

                    //Add them to the local
                    Context.listOfLocalDirectories.Add(downloadToDirectory);
                }         
            }
            catch (Exception)
            {
                Errors.printError("Failed to download File: " + value.docName);
            }
        }
    }

Answer 1

在DownloadFileAsync/ DownloadFileCompleted的成员WebClient使用基于事件的异步模式.如果要使用async和await,则应使用基于任务的异步模式.

在这种情况下,您应该使用该DownloadFileTaskAsync成员,如下:

private async Task DownloadFileAsync(DocumentObject doc)
{
  try
  {
    using (WebClient webClient = new WebClient())
    {
      string downloadToDirectory = @Resources.defaultDirectory + doc.docName;
      webClient.Credentials = System.Net.CredentialCache.DefaultNetworkCredentials;
      await webClient.DownloadFileTaskAsync(new Uri(doc.docUrl), @downloadToDirectory);

      //Add them to the local
      Context.listOfLocalDirectories.Add(downloadToDirectory);
    }         
  }
  catch (Exception)
  {
    Errors.printError("Failed to download File: " + doc.docName);
  }
}

private async Task DownloadMultipleFilesAsync(List<DocumentObject> doclist)
{
  await Task.WhenAll(doclist.Select(doc => DownloadFileAsync(doc)));
}

请注意,您Context.listOfLocalDirectories.Add和Errors.printError方法应该是线程安全的.