ImageBrush for Grid 以编程方式提供

所以我尝试应用图像，但看不到任何更改...

我缺少什么？谢谢！！

BitmapImage bi = new BitmapImage();
bi.BeginInit();
bi.UriSource = new Uri(@"pack://application:,,,/Images/bg1.jpg", UriKind.RelativeOrAbsolute);
bi.EndInit();
ImageBrush ib = new ImageBrush();
ib.TileMode = TileMode.Tile;
ib.ImageSource = bi;
ib.Stretch = Stretch.None;
RootGrid.Background = ib;

var ib = new ImageBrush {
  ImageSource =
    new BitmapImage(
      new Uri(@"Images'bg1.jpg", UriKind.Relative)
    )
};
RootGrid.Background = ib;