Cancel WebClient DownloadTask Async

Question

I have a WebClient which I use to download a file.
This is my code, in which I have a ProgressDialog and a WebClient to download:

dialog = new ProgressDialog(mContext);
                dialog.SetProgressStyle(Android.App.ProgressDialogStyle.Horizontal);
                dialog.SetCancelable(true);
                dialog.SetCanceledOnTouchOutside(true);

                dialog.Show();// showing a dialog

                string url = "myurl";

                WebClient webClient = new WebClient();
                webClient.DownloadProgressChanged += WebClient_DownloadProgressChanged;
                dialog.CancelEvent += (s, e) =>
                {
                    webClient.CancelAsync();
       //----------------------- Crashes Here
                };

                try
                {
                    bytes = await webClient.DownloadDataTaskAsync(url);
                }
                catch (TaskCanceledException)
                {
                    return;
                }
                catch (Exception a)
                {
                    return;
                }

How do I cancel the download in the middle?

webClient.CancelAsync();

throws an exception:

Object reference not set to an instance of an object

@StephenCleary "System.NullReferenceException: Object reference not set to an instance of an object" — amitairos
– amitairos, Commented Jul 3, 2017 at 13:11
Have you tried to use a private variable for webClient or better a private locked singleton ? — Jimbot
– Jimbot, Commented Jul 3, 2017 at 13:40

Yuri S · Accepted Answer · 2017-07-04 21:06:45Z

The problem was in exception handling code when inner exception was null. To make it work just checking for inner exception using "?"

dialog.CancelEvent += (s, e) =>
                    {
                        webClient.CancelAsync();
                    };

                    try
                    {
                        bytes = await webClient.DownloadDataTaskAsync(url);
                    }
                    catch (WebException wex)
                    {
                        if (wex.Status == WebExceptionStatus.RequestCanceled)
                            return;
                        Toast.MakeText(mContext, wex.Message + "," + wex?.InnerException?.Message, ToastLength.Long).Show();
                        dialog.Progress = 0;
                        return;
                    }
                    catch (TaskCanceledException)
                    {
                        return;
                    }
                    catch (Exception a)
                    {
                        Toast.MakeText(mContext, a.Message + "," + a?.InnerException?.Message, ToastLength.Long).Show();
                        dialog.Progress = 0;
                        return;
                    }

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