下載多個文件異步並等待它們全部完成，然後執行代碼的其餘部分

Question

我試圖從互聯網上下載多個文件並等待所有文件完成。這是我正在運行的C＃控制檯應用程序，因此不需要進度條事件處理程序。但是，即使所有文件都沒有被下載，它目前只是繼續執行代碼。

• 1.下載所有文件！
• 2.Finished下載文件A
• 3.Finished下載所有文件！
• 4.Finished下載文件B
• 5.Finished下載文件C

你會如何等待，直到所有異步下載文件完成。

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

的WebClient的DownloadFileAsync/DownloadFileCompleted成員使用Event-based Asynchronous Pattern。如果你想使用async和await，你應該使用Task-based Asynchronous Pattern。

在這種情況下，你應該使用DownloadFileTaskAsync member，因爲這樣的：

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))); 
} 

請注意，您的Context.listOfLocalDirectories.Add和Errors.printError方法應該是線程安全的。