请问我做的后台下载为啥直到下载完成前没法获取文件总大小呢

[weixin_38064768]

BackgroundDownloader downloader = new BackgroundDownloader(); StorageFile file = await KnownFolders.MusicLibrary.CreateFileAsync("1.mp3",CreationCollisionOption.ReplaceExisting); download = downloader.CreateDownload(new Uri("http://caige.uwp.store/AppAssets/Music/1.mp3"), file); await download.StartAsync(); GameData.ShowBaiFenBi = ThreadPoolTimer.CreatePeriodicTimer(async(T) => { if (download.Progress.TotalBytesToReceive > 0) { double d = download.Progress.BytesReceived / download.Progress.TotalBytesToReceive * 100; double baiFenBi = Double.Parse(d.ToString("F1")); await Dispatcher.RunAsync(CoreDispatcherPriority.Normal, () => { Downloading_ProgressBar.Value = baiFenBi; }); } }, TimeSpan.FromMilliseconds(500));
请问为啥直到下载完成前没法获取文件总大小

因为没法获取总大小的关系，下载完成前没法提供进度