C#生成日期流水账号

生成类似 LS14120002       
 private String GenNewOrder()
        {
            String newOrder;
            var orders = LYOrder.Items;
            String lastOrder = ((Order)orders[orders.Count - 1]).OrderNum;
            //int len = lastOrder.Length;
            String dateOld = lastOrder.Substring(2, 4);
            int indexOld = Convert.ToInt32(lastOrder.Substring(6, 4));
            String indexNew = (indexOld + 1).ToString();

            String dateNow = System.DateTime.Now.ToString("yyMM");//获取年月
            if (dateOld.Equals(dateNow))//同一个月的单
            {

                switch (indexNew.Length)
                {
                    case 1:
                        indexNew = "000" + indexNew;
                        break;
                    case 2:
                        indexNew = "00" + indexNew;
                        break;
                    case 3:
                        indexNew = "0" + indexNew;
                        break;
                }
                newOrder = " LS" + dateNow + indexNew;

            }
            else
            {
                newOrder = " LS" + dateNow + "0001";
            }

            return newOrder;

        }