BackgroundDownloader系统找不到指定的文件

Question

我使用C＃和Windows Phone 8.1作为通用。

我遇到BackgroundDownloader问题。当我开始下载时，它给了我一个例外：

Downloading http://localhost/MySong.mp3 to MySong.mp3 with Default priority, a95c00db-738d-4e22-a456-dc30d49b0a3b


A first chance exception of type 'System.IO.FileNotFoundException' occurred in mscorlib.dll
Exception: The system cannot find the file specified. (Exception from HRESULT: 0x80070002)

这是我的代码：

        private void downladButton_Tapped(object sender, TappedRoutedEventArgs e)
        {
            PumpUrl(txtUrl.Text);
        }
        async void PumpUrl(string url)
        {
            if (!string.IsNullOrEmpty(url) && CheckUrl(url))
                StartDownloadMethod(url);
            else
                await new MessageDialog("Looks like URL is invalid.\r\nPlease check your URL and try again"").ShowAsync();
        }
        public bool CheckUrl(string URL)
        {
            Uri source;
            if (Uri.TryCreate(URL, UriKind.RelativeOrAbsolute, out source))
                return true;
            else
                return false;
        }
        public void StartDownloadMethod(string url, string fileName = null)
        {
            StartDownload(url, BackgroundTransferPriority.Default, false, fileName);
        }

        private async void StartDownload(string url, BackgroundTransferPriority priority, bool requestUnconstrainedDownload, string fileName = null)
        {
            Uri source;
            if (!Uri.TryCreate(url.Trim(), UriKind.RelativeOrAbsolute, out source))
            {
                Debug.WriteLine("Invalid URI.");
                return;
            }

            string destination = null;

            if (string.IsNullOrEmpty(fileName))
                try
                {
                    destination = System.IO.Path.GetFileName(Uri.UnescapeDataString(url)).Trim();
                }
                catch { }
            else
                destination = Uri.UnescapeDataString(fileName).Trim();


            if (string.IsNullOrWhiteSpace(destination))
            {
                Debug.WriteLine("A local file name is required.");
                return;
            }

            StorageFile destinationFile = null;
            try
            {
                if (vars.localType == LocalType.SDCard)
                    destinationFile = await KnownFolders.RemovableDevices.CreateFileAsync(
    "Downloads\\Test App\\" + destination, CreationCollisionOption.GenerateUniqueName);
                else
                    destinationFile = await KnownFolders.MusicLibrary.CreateFileAsync(
"Test App\\" + destination, CreationCollisionOption.GenerateUniqueName);
            }
            catch (FileNotFoundException ex)
            {
                Debug.WriteLine("Error while creating file: " + ex.Message);
                return;
            }
            catch (Exception ex) { Debug.WriteLine("Error while creating file: " + ex.Message); }

            if (destinationFile == null)
                return;


            BackgroundDownloader downloader = new BackgroundDownloader();
            DownloadOperation download = downloader.CreateDownload(source, destinationFile);

            Debug.WriteLine(String.Format(CultureInfo.CurrentCulture, "Downloading {0} to {1} with {2} priority, {3}",
                source.AbsoluteUri, destinationFile.Name, priority, download.Guid));

            download.Priority = priority;
            Add(url, destination, download);
            if (!requestUnconstrainedDownload)
            {
                // Attach progress and completion handlers.
                await HandleDownloadAsync(download, true);
                return;
            }

            List<DownloadOperation> requestOperations = new List<DownloadOperation>();
            requestOperations.Add(download);

            UnconstrainedTransferRequestResult result;
            try
            {
                result = await BackgroundDownloader.RequestUnconstrainedDownloadsAsync(requestOperations);
            }
            catch (NotImplementedException)
            {
                Debug.WriteLine(
                    "BackgroundDownloader.RequestUnconstrainedDownloadsAsync is not supported in Windows Phone.");
                return;
            }

            Debug.WriteLine(String.Format(CultureInfo.CurrentCulture, "Request for unconstrained downloads has been {0}",
                (result.IsUnconstrained ? "granted" : "denied")));

            await HandleDownloadAsync(download, true);
        }

        private async Task HandleDownloadAsync(DownloadOperation download, bool start)
        {
            try
            {
                Debug.WriteLine("Running: " + download.Guid);
                bool bb = false;
                foreach (DownloadOperation item in DownloadDB.activeDownloads)
                {
                    if (item != null && item.Guid == download.Guid)
                    {
                        bb = true;
                        break;
                    }
                }
                if (!bb)
                    DownloadDB.activeDownloads.Add(download);

                Progress<DownloadOperation> progressCallback = new Progress<DownloadOperation>(DownloadProgress);
                CancellationTokenSource cts = new CancellationTokenSource();
                AppendCancellationTokenSource(download, cts);

                if (start)
                    await download.StartAsync().AsTask(cts.Token, progressCallback);
                else
                    await download.AttachAsync().AsTask(cts.Token, progressCallback);

                ResponseInformation response = download.GetResponseInformation();

                DownloadDB.RemoveChildByGuid(download.Guid.ToString());
                DownloadDB.SaveDB();
                Debug.WriteLine(String.Format(CultureInfo.CurrentCulture, "Completed: {0}, Status Code: {1}",
                    download.Guid, response.StatusCode));

            }
            catch (TaskCanceledException taskCanceled)
            {
                Debug.WriteLine("TaskCanceledException: " + download.Guid + "\t" + taskCanceled.Message);
                DownloadDB.RemoveChildByGuid(download.Guid.ToString());
            }
            catch (Exception ex)
            {
                Debug.WriteLine("Exception: " + ex.Message);
            }
            finally
            {
                DownloadDB.RemoveChildByGuid(download.Guid.ToString());
                DownloadDB.activeDownloads.Remove(download);
            }
        }

我在3个月前在其他应用程序中使用了我的代码，它工作正常，但在此应用程序中工作。哪里错了？

由于