取消WebClient DownloadTask異步

Question

我有一個WebClient，我用它來下載文件。
這是我的代碼，其中我有一個ProgressDialog和Web客戶端下載：

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; 
       } 

如何取消下載在中間？

webClient.CancelAsync(); 

拋出異常：

Object reference not set to an instance of an object 

Answer 1

的問題是在異常處理代碼當內部異常爲空。爲了使它工作只是使用「？」來檢查內部異常

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; 
        }