public class BillNumberBuilder{    private static object locker = new object();         private static int sn = 0;         public static string NextBillNumber(){        lock(locker){            if(sn == 9999999999)                sn = 0;            else                sn++;            return DateTime.Now.ToString("yyyyMMddHHmmss") + sn.ToString().PadLeft(10, ‘0‘);        }    }    // 防止创建类的实例    private BillNumberBuilder(){}}