Question

Can someone explain to me why in .NET 2.0 if I have an interface, IPackable and a class that implements that interface OrderItem, when I have a method that takes in a List<IPackable>, passing in a list of List<OrderItem> does not work?

Does anyone know how I could accomplish this functionality?

Code:

public interface IPackable {         double Weight{ get; } }  public class OrderItem : IPackable   public List<IShipMethod> GetForShipWeight(List<IPackable> packages) {    double totalWeight = 0;    foreach (IPackable package in packages) {         totalWeight += package.Weight;    } } 

The following code does not work.

List<OrderItem> orderItems = new List<OrderItem>(); List<IShipMethod> shipMethods = GetForShipWeight(orderItems); 

Answer 1

The feature is called covariance/contravariance and will be supported in c# 4.0. You can read about it here: http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx