.NET Casting Generic List

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);

Solution 1:

JMD is half correct. In fact, it's absolutely incorrect to say that we will be able to cast a generic list with C# 4.0. It's true that covariance and contravariance will be supported in C# 4.0 but it will only works with interface and delegate and there will have a lot of constraints. Therefore, it won't work with List.

The reason is really simple.

If B is a subclass of A, we cannot say that List<B> is a subclass of List<A>.

And here's why.

List<A> exposes some covariances methods (returning a value) and some contravariances methods (accepting a value as a parameter).

e.g.

• List<A> exposes Add(A);
• List<B> exposes Add(B);

If List<B> inherits from List<A>...than you would be able to do List<B>.Add(A);

Therefore, you would loose all type safety of generics.

Solution 2:

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

Solution 3:

JMD's answer is correct. For a workaround, you can try this:

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

Or, if the list must be strongly typed as OrderItems, then this (3.0 only, sorry):

List<IShipMethod> shipMethods =
    GetForShipWeight(orderItems.Cast<IPackable>().ToList());