UWP System.UnauthorizedAccessException：拒絕訪問路徑

Question

using(WebClient cln = new WebClient()) {
    try {
        FileSavePicker picker = new FileSavePicker();
        picker.FileTypeChoices.Add("PNG File", new List < string > () {
            ".png"
        });
        picker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;

        var file = await picker.PickSaveFileAsync();

        cln.DownloadFile("https://i.redd.it/o8rz4s0lxp021.png", file.Path);
    } catch (Exception e) {
        Debug.WriteLine(e);
    }
}

當我開始創建這個文件時，它拋出 System.UnauthorizedAccessException 並且文件已損壞。 任何想法有什么問題？

Answer 1

看來你這里有同步問題。 您應該確保在下載內容之前完成FileSavePicker作業，因為您正在嘗試異步更新文件（ await picker.PickSaveFileAsync(); ）。 為此，您可以使用FileUpdateStatus

using (WebClient cln = new WebClient())
{
    try
    {
        FileSavePicker picker = new FileSavePicker();
        picker.FileTypeChoices.Add("PNG File", new List<string>() { ".png" });
        picker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;

        var file = await picker.PickSaveFileAsync();
          // Prevent updates to the remote version of the file until we finish making changes and call CompleteUpdatesAsync.
        CachedFileManager.DeferUpdates(file);
        // write to file
        await FileIO.WriteTextAsync(file, file.Name);
        // Let Windows know that we're finished changing the file so the other app can update the remote version of the file.
        // Completing updates may require Windows to ask for user input.
        FileUpdateStatus status = await CachedFileManager.CompleteUpdatesAsync(file);
        if (status == FileUpdateStatus.Complete)
        {
            cln.DownloadFile("https://i.redd.it/o8rz4s0lxp021.png", file.Path);
        }
    }
    catch (Exception e)
    {
        Debug.WriteLine(e);
    }
}

Answer 2

好的，我通過改變這個來修復它：

cln.DownloadFile("https://i.redd.it/o8rz4s0lxp021.png", file.Path); 到背景下載器：

BackgroundDownloader downloader = new BackgroundDownloader();
DownloadOperation download = downloader.CreateDownload(new Uri("https://i.redd.it/o8rz4s0lxp021.png"), file);
await download.StartAsync();