public Customer Add(Customer customer)
        {
            using (var context = new DemoContextFactory().Create())
            {
                _customers.Add(customer);
                context.SaveChanges();
            }

            //todo: add Id field
            return(customer);
        }
        public Customer Delete(int id)
        {
            Customer itemToRemove;

            using (var context = new DemoContextFactory().Create())
            {
                _customers = context.Customers.ToList();

                itemToRemove = _customers.SingleOrDefault(c => c.CustomerId == id);

                if (itemToRemove != null)
                {
                    //_customers.Remove(itemToRemove);
                    context.Customers.Remove(itemToRemove);
                    context.SaveChanges();
                }
            }
            return(itemToRemove);
        }