protected override void OnPreRender(EventArgs e)
    {
        base.OnInit(e);

        GridView extendee = 
            this.NamingContainer.FindControl(this.ExtendeeID) as GridView;
        if (extendee != null && 
            extendee.AllowSorting && 
            extendee.HeaderRow != null && 
            !String.IsNullOrEmpty(extendee.SortExpression))
        {
            int field = GetSortField(extendee);
            if (field >= 0)
            {
                Image img = new Image();
                img.ImageUrl = 
                    extendee.SortDirection == SortDirection.Ascending ? 
                        this.AscendingImageUrl : this.DescendingImageUrl;
                img.ImageAlign = ImageAlign.TextTop;

                extendee.HeaderRow.Cells[field].Controls.Add(img);
            }
        }
    }