tx.cs

public class Invoice
{
    public string InvoiceNumber { get; set; }
    public Customer Customer { get; set; }
    public List<Product> Items { get; set; }
}

public class Customer
{
    public string Name { get; set; }
    public string Street { get; set; }
    public int Zip { get; set; }
    public string City { get; set; }
}

public class Product
{
    public string Name { get; set; }
    public string Description { get; set; }
}