协方差在C＃ [英] covariance in c#

问题描述

是否有可能蒙上了列表＆LT;子类化＆GT; 到列表＆LT;超类＆GT; 在C＃4.0吗？

Is it possible to cast a List<Subclass> to List<Superclass> in C# 4.0?

沿着这些路线的内容：

class joe : human {}

List<joe> joes = GetJoes();

List<human> humanJoes = joes;

这不是什么协方差是？

Isn't this what covariance is for?

如果你可以这样做：

human h = joe1 as human;

为什么你不应该是能够做到

why shouldn't you be able to do

List<human> humans = joes as List<human>; 

比它不会是合法的情况（JOE）人类[0]，因为该项目已下降铸造..和每个人都会很高兴。现在唯一的方法是创建一个新的List

than it wouldn't be legal to do (joe)humans[0] because that item has been down casted.. and everyone would be happy. Now the only alternative is to create a new List

推荐答案

您不能做到这一点，因为它不会是安全的。试想一下：

You can't do this, because it wouldn't be safe. Consider:

List<Joe> joes = GetJoes();    
List<Human> humanJoes = joes;
humanJoes.Clear();
humanJoes.Add(new Fred());
Joe joe = joes[0];

很明显，最后一行（如果不是更早的一个）的的的失败 - 作为一个弗雷德不是乔。 清单1所述的不变性; T＆GT; prevents这个错误在编译的时间，而不是执行时间

Clearly the last line (if not an earlier one) has to fail - as a Fred isn't a Joe. The invariance of List<T> prevents this mistake at compile time instead of execution time.

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