如何控制网格的Z顺序

        TextBlock tmp = new TextBlock
        {
            Text = displayText,
            Foreground = new SolidColorBrush(Colors.Red),
            HorizontalAlignment = HorizontalAlignment.Center,
            VerticalAlignment = VerticalAlignment.Center,
            FontSize = 30
        };
        Grid grd = new Grid();
        grd.Children.Add(tmp);
        // grd.Background = new SolidColorBrush(Colors.LightGray);
        Viewbox vb = new Viewbox();
        vb.Child = grd;
        vb.Width = width;
        vb.Height = height;
        DrawingCvs.Children.Add(vb);
        Canvas.SetLeft(vb, xpos);
        Canvas.SetTop(vb, ypos);
        Canvas.SetZIndex(grd, 1000);

        // we need a second grid for cosmetic reasons. Due to
        // how the viewbox works, when we resize we lose the full
        // width of the grid which has a textblock in it
        Grid cosmeticGrd = new Grid
                          {
                              Width = width,
                              Height = height,
                              Background = new SolidColorBrush(Colors.LightGray)
                          };
        DrawingCvs.Children.Add(cosmeticGrd);
        Canvas.SetLeft(cosmeticGrd, xpos);
        Canvas.SetTop(cosmeticGrd, ypos);
        Canvas.SetZIndex(grd, 0);