取消WebClient DownloadTask Async

Question

我有一个用于下载文件的 WebClient。
这是我的代码，其中我有一个 ProgressDialog 和一个要下载的 WebClient：

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;
                }

如何中途取消下载？

webClient.CancelAsync();

抛出异常：

Object reference not set to an instance of an object

Answer 1

当内部异常为空时，问题出在异常处理代码中。要使其正常工作，只需使用“?”

检查内部异常

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;
                    }