有谁知道我怎么能完成这个功能？

码：

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;
   }
}

以下代码不起作用.

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