test.cs

public class Order
{
    public int OrderID { get; set; }
    public Customer Customer { get; set; }
    public List<OrderItem> OrderItems { get; set; }
}

public class OrderItem
{
    public Product Product { get; set; }
    public int Quantity { get; set; }
}

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

public class Customer
{
    public string Name { get; set; }
}