c# - 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.