拍摄一张照片，并将其保存到Windows 8 Metro风格的应用程序中的磁盘上

我可以拍照，但在保存到KnownFolders时遇到问题。是的，我已经在Package.appxmanifest.中声明了图片库访问能力

var ui = new CameraCaptureUI();
ui.PhotoSettings.CroppedAspectRatio = new Size(4, 3);
StorageFile file = await ui.CaptureFileAsync(CameraCaptureUIMode.Photo);
if (file != null)
{
    var stream = await file.OpenAsync(FileAccessMode.Read);
    var bitmap = new BitmapImage();
    bitmap.SetSource(stream);
    Photo.Source = bitmap;
    StorageFolder storageFolder = KnownFolders.PicturesLibrary;
    var result = await file.CopyAsync(storageFolder, "tps.jpg");
}

代码在最后一行停止。我做错了什么？