Download multiple files async and wait for all of them to finish before executing the rest of the code

Question

I am trying to download multiple files from the internet and await for all of them to finish. This is a C# console application that I am running, so no progress bar event handler should be necessary. However it currently just continues to execute code even though all files have not been downloaded.

• 1.Downloading all files!
• 2.Finished Download File A
• 3.Finished Downloading all files!
• 4.Finished Downloading File B
• 5.Finished Downloading File C

How would you await till all async download files are finished.

 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

The DownloadFileAsync/DownloadFileCompleted members of WebClient use the Event-based Asynchronous Pattern. If you want to use async and await, you should be using the Task-based Asynchronous Pattern.

In this case, you should use the DownloadFileTaskAsync member, as such:

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))); } 

Please note that your Context.listOfLocalDirectories.Add and Errors.printError methods should be threadsafe.