在C＃4.0泛型方差 [英] Generic Variance in C# 4.0

问题描述

一般差异在C＃4.0已经以这样的方式来实现，它可以编写以下无异常（这是在C＃中会发生什么3.0）：

Generic Variance in C# 4.0 has been implemented in such a way that it's possible to write the following without an exception (which is what would happen in C# 3.0):

 List<int> intList = new List<int>();
 List<object> objectList = intList; 

【例非功能性：参见乔恩斯基特的答案] 的

[Example non-functional: See Jon Skeet's answer]

我最近参加了一个会议，其中乔恩斯基特给通用差异的一个很好的概述，但我不知道我完全得到它 - 我理解的的意义和退出键的话，当谈到禁忌和共同方差，但我很好奇，在幕后发生的事情。

I recently attended a conference where Jon Skeet gave an excellent overview of Generic Variance, but I'm not sure I'm completely getting it - I understand the significance of the in and out key words when it comes to contra and co-variance, but I'm curious to what happens behind the scenes.

什么CLR的看到，当这code执行是它隐含转换名单，其中; INT＆GT; 到名单，其中，对象＆gt; 或者它只是内置的，我们现在可以派生类型之间进行转换，以父类型

What does the CLR see when this code is executed? Is it implicitly converting the List<int> to List<object> or is it simply built in that we can now convert between derived types to parent types?

出于兴趣，为什么不这样介绍了previous版本，什么是主要的好处 - 即现实世界中使用

更多信息<一个href="http://stackoverflow.com/questions/245607/how-is-generic-covariance-contra-variance-implemented-in-c-4-0">post对于通用方差（但问题是非常过时，寻找真正的，最新的最新信息）

More info on this post for Generic Variance (but question is extremely outdated, looking for real, up-to-date information)

推荐答案

没有，你的榜样也不会，原因有三工作：

No, your example wouldn't work for three reasons:

• 类（如名单，其中，T＆GT; ）是不变的;只有代表和接口都变
• 方差工作，接口只使用类型参数在一个方向（为逆变，出来的协方差）
• 在值类型不支持类型参数的变化 - 因此没有从的IEnumerable＆LT converstion; INT＆GT; 到的IEnumerable＆LT;对象＆gt; 例如

• Classes (such as List<T>) are invariant; only delegates and interfaces are variant
• For variance to work, the interface has to only use the type parameter in one direction (in for contravariance, out for covariance)
• Value types aren't supported as type arguments for variance - so there's no converstion from IEnumerable<int> to IEnumerable<object> for example

（将code编译失败在C＃3.0和4.0 - 没有例外）

(The code fails to compile in both C# 3.0 and 4.0 - there's no exception.)

所以这个的会的工作：

IEnumerable<string> strings = new List<string>();
IEnumerable<object> objects = strings;

在CLR只是使用参考，不变的 - 没有创建新的对象。所以，如果你叫 objects.GetType（）你还能名单，其中，串＆GT;

我相信这不是前面介绍，因为语言设计者仍然必须制定出如何暴露的细节 - 它一直以来V2的CLR

I believe it wasn't introduced earlier because the language designers still had to work out the details of how to expose it - it's been in the CLR since v2.

的好处是相同的要在其中能够使用一种类型作为另一个其它时间。要使用相同的例子，我用最后一个星期六，如果你已经得到的东西器具的IComparer＆LT;形状＆GT; 按面积来比较的形状，它的疯狂，你不能用它来排序名单，其中，圈＆GT; - 如果它可以比较任意两个形状，它当然可以比较任意两个圆圈。由于C＃4，会从的IComparer℃的逆变转换;形状＆GT; 到的IComparer＆LT;圈＆GT; 所以你可以调用 circles.Sort（areaComparer）。

The benefits are the same as other times where you want to be able to use one type as another. To use the same example I used last Saturday, if you've got something implements IComparer<Shape> to compare shapes by area, it's crazy that you can't use that to sort a List<Circle> - if it can compare any two shapes, it can certainly compare any two circles. As of C# 4, there'd be a contravariant conversion from IComparer<Shape> to IComparer<Circle> so you could call circles.Sort(areaComparer).

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