Setting Tab in Gridview row wise

protected void Gridview1_RowDataBound(object sender, GridViewRowEventArgs e)
    {
        if (e.Row.RowType == DataControlRowType.DataRow)
        {
            DropDownList ddl = e.Row.FindControl("txtdes") as DropDownList;
            TextBox txt1 = e.Row.FindControl("txtquan") as TextBox;
            TextBox txt2 = e.Row.FindControl("ddlDay") as TextBox;
            TextBox txt3 = e.Row.FindControl("txtTotal") as TextBox;
            int i = e.Row.RowIndex * 4;
            ddl.Attributes.Add("tabindex", (i + 1).ToString());
            txt1.Attributes.Add("tabindex", (i + 2).ToString());
            txt2.Attributes.Add("tabindex", (i + 3).ToString());
            txt3.Attributes.Add("tabindex", (i + 4).ToString());
            if (e.Row.RowIndex == 0)
                ddl.Focus();
        }
    }