public List<Customer> Retrieve()
        {
            InvoiceRepository invoiceRepository = new InvoiceRepository();

            List<Customer> custList = new List<Customer>
                    {new Customer()
                          { CustomerId = 1,
                            FirstName="Frodo",
                            LastName = "Baggins",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=1,
                            InvoiceList=invoiceRepository.Retrieve(1)},
                    new Customer()
                          { CustomerId = 2,
                            FirstName="Bilbo",
                            LastName = "Baggins",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=null,
                            InvoiceList=invoiceRepository.Retrieve(2)},
                    new Customer()
                          { CustomerId = 3,
                            FirstName="Samwise",
                            LastName = "Gamgee",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=4,
                            InvoiceList=invoiceRepository.Retrieve(3)},
                    new Customer()
                          { CustomerId = 4,
                            FirstName="Rosie",
                            LastName = "Cotton",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId = 2,
                            InvoiceList = invoiceRepository.Retrieve(4)},
                    new Customer()
                          { CustomerId = 3,
                            FirstName="Samwise",
                            LastName = "Gamgee",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=4,
                            InvoiceList=invoiceRepository.Retrieve(3)},
                    new Customer()
                          { CustomerId = 3,
                            FirstName="Samwise",
                            LastName = "Gamgee",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=4,
                            InvoiceList=invoiceRepository.Retrieve(3)},
                    new Customer()
                          { CustomerId = 3,
                            FirstName="Samwise",
                            LastName = "Gamgee",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=4,
                            InvoiceList=invoiceRepository.Retrieve(3)},
                    new Customer()
                          { CustomerId = 3,
                            FirstName="Samwise",
                            LastName = "Gamgee",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=4,
                            InvoiceList=invoiceRepository.Retrieve(3)},
                    new Customer()
                          { CustomerId = 3,
                            FirstName="Samwise",
                            LastName = "Gamgee",
                            EmailAddress = "*****@*****.**",
                            CustomerTypeId=4,
                            InvoiceList=invoiceRepository.Retrieve(3)}};

            return custList;
        }