public static int Registe(string firstName, string lastName, string phone, string city)
        {
            int ID = -1;

            //check if the customer already existing
            Customers.ForEach(c =>
            {
                ID = (c.FirstName == firstName && c.LastName == lastName) ? c.ID : ID;
            });

            //new customer, create record and return new ID.
            if (ID == -1)
            {
                Customer newCus = new Customer()
                {
                    FirstName = firstName,
                    LastName  = lastName,
                    Phone     = phone,
                    City      = city
                };
                try
                {
                    ID = GenericDB.GenericInsert <Customer>("Customer", newCus);
                } catch (Exception ex)
                {
                    throw new Exception(ex.Message, ex);
                }
            }


            return(ID);
        }
        /// <summary>
        /// add new lease to DB
        /// </summary>
        /// <param name="customerID"></param>
        /// <param name="slipId"></param>
        public static void addLease(int customerID, int slipId)
        {
            Lease newLease = new Lease()
            {
                CustomerID = customerID,
                SlipID     = slipId,
            };

            try
            {
                GenericDB.GenericInsert <Lease>("Lease", newLease);
                Leases = GenericDB.GenericRead <Lease>("Lease");//Static Leases, so need update Leases after each insert.
            }
            catch (Exception ex)
            {
                throw new Exception(ex.Message, ex);
            }
        }