public class Customer
    {
        public int CustomerID { get; set; }
        public string Name { get; set; }
        public ICollection<Account> Accounts { get; set; }
    }

    public class Account
    {
        public int AccountID { get; set; }
    }

    public interface ICustomerRepository
    {
        IEnumerable<Customer> GetCustomers();
        Customer GetCustomerByID(int customerID);
        void InsertCustomer(Customer customer);
        void UpdateCustomer(Customer customer);
        void DeleteCustomer(int customerID);
    }

    public class CustomerRepository : ICustomerRepository
    {
        public IEnumerable<Customer> GetCustomers()
        {
            List<Customer> customers = new List<Customer>();
            customers.Add(new Customer { CustomerID = 1, Name = "Customer1" });
            customers.Add(new Customer { CustomerID = 2, Name = "Customer2" });
            customers.Add(new Customer { CustomerID = 3, Name = "Customer3" });
            customers.Add(new Customer { CustomerID = 4, Name = "Customer4" });
            customers.Add(new Customer { CustomerID = 5, Name = "Customer5" });

            return customers;
        }

        public Customer GetCustomerByID(int customerID)
        {
            throw new NotImplementedException();
        }

        public void InsertCustomer(Customer customer)
        {
            throw new NotImplementedException();
        }

        public void UpdateCustomer(Customer customer)
        {
            throw new NotImplementedException();
        }

        public void DeleteCustomer(int customerID)
        {
            throw new NotImplementedException();
        }
    }

    public class Customer
    {
        public int CustomerID { get; set; }
        public string Name { get; set; }
        public Lazy<List<Account>> Accounts { get; set; }
    }

    public class CustomerRepository : ICustomerRepository
    {
        public IEnumerable<Customer> GetCustomers()
        {
            List<Customer> customers = new List<Customer>();
            customers.Add(new Customer
            {
                CustomerID = 1,
                Name = "Customer1",
                Accounts = new Lazy<List<Account>>(() => { return GetAccountByCustomerID(1); })
            });
            customers.Add(new Customer
            {
                CustomerID = 2,
                Name = "Customer2",
                Accounts = new Lazy<List<Account>>(() => { return GetAccountByCustomerID(2); })
            });
            customers.Add(new Customer
            {
                CustomerID = 3,
                Name = "Customer3",
                Accounts = new Lazy<List<Account>>(() => { return GetAccountByCustomerID(3); })
            });
            customers.Add(new Customer
            {
                CustomerID = 4,
                Name = "Customer4",
                Accounts = new Lazy<List<Account>>(() => { return GetAccountByCustomerID(4); })
            });
            customers.Add(new Customer
            {
                CustomerID = 5,
                Name = "Customer5",
                Accounts = new Lazy<List<Account>>(() => { return GetAccountByCustomerID(5); })
            });

            return customers;
        }

        private List<Account> GetAccountByCustomerID(int customerID)
        {
            List<Account> list = new List<Account>();
            list.Add(new Account { AccountID = 100, CustomerID = customerID });
            list.Add(new Account { AccountID = 200, CustomerID = customerID });

            return list;
        }
    }