test.cs

public class Invoice {
  public Buyer Buyer { get; set; }
  public List<LineItem> LineItems { get; set; }
  public float Total {
     get {
        var total = 0F;

        foreach (var item in LineItems) {
           total += item.LineTotal;
        }

        return total;
     }
  }
}

public class LineItem {
  public string ProductID { get; set; }
  public string Name { get; set; }
  public int Quantity { get; set; }
  public float Price { get; set; }
  public float LineTotal {
     get { return Price * Quantity; }
  }
}

public class Buyer {
  public string Name { get; set; }
  public string ContactName { get; set; }
  public string Street { get; set; }
  public string City { get; set; }
}