.NET铸造通用列表 [英] .NET Casting Generic List

问题描述

有人可以解释为什么在.NET 2.0如果我有一个接口， IPackable 和实现该接口的类 OrderItem List< IPackable> ，传递 List< OrderItem> 不工作？

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?

有没有人知道如何完成这项功能？

Does anyone know how I could accomplish this functionality?

代码：

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;
   }
}

以下代码无效。

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

推荐答案

此功能称为协方差/逆变在c＃4.0。您可以在这里阅读： http：// blogs .msdn.com / ericlippert / archive / tags /协变度+和+ Contravariance / default.aspx

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

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