.NET铸造泛型列表 [英] .NET Casting Generic List

问题描述

有人能向我解释为什么在.NET 2.0，如果我有一个接口， IPackable 和一个类实现该接口订单项，当我有一个方法，需要在名单，其中，IPackable＆GT; ，传递的名单名单，其中，订单项＆GT; 不工作？

有谁知道我能做到这一点的功能？

code：

 公共接口IPackable {
        双重量{获得; }
}

公共类订单项：IPackable


公开名单＆LT; IShipMethod＆GT; GetForShipWeight（名单＆LT; IPackable＆GT;包）{
   双totalWeight = 0;
   的foreach（IPackable包套餐）{
        totalWeight + = package.Weight;
   }
}
 

下面code不起作用。

 名单，其中，订单项＆GT;的OrderItems =新的名单，其中，订单项＆GT;（）;
名单＆LT; IShipMethod＆GT; shipMethods = GetForShipWeight（的OrderItems）;
 

解决方案

该功能被称为协方差/逆变，将在C＃4.0的支持。你可以在这里阅读有关：<一href="http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx">http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx

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);

解决方案

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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