Sun Changing? DrawingVisual Fl? Surface and contour

I have a DrawingVisual object and I want to change its fill and its stroke.

I’ve tried this for Fill:

DrawingVisual MyDrawing = new DrawingVisual();
SetFill(Brushes.Red, MyDrawing.Drawing);

Where SetFill is:

private void SetFill(Brush fill, DrawingGroup group)
{
    foreach (Drawing drw in group.Children)
    {
        if (drw is DrawingGroup)
            SetFill(fill, drw as DrawingGroup);
        else if (drw is GeometryDrawing)
        {
            GeometryDrawing geo = drw as GeometryDrawing;
            geo.Brush = fill;

            using (DrawingContext context = MyDrawing.RenderOpen())
            {
                context.DrawDrawing(group);
            }
        }
    }
}

But in this way may happen that my DrawingVisual is drawn to a different position, as if the transformations have not been applied more (to MyDrawing).

How can I do?
Thanks.