如何在 C# 4.0 中实现通用协方差和逆变？
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How is Generic Covariance & Contra-variance Implemented in C# 4.0?
提问by Morgan Cheng
I didn't attend PDC 2008, but I heard some news that C# 4.0 is announced to support Generic covariance and contra-variance. That is, List<string>can be assigned to List<object>. How could that be?
我没有参加 PDC 2008，但我听说 C# 4.0 宣布支持通用协变和逆变的消息。也就是说，List<string>可以赋值给List<object>。怎么会这样？
In Jon Skeet's book C# in Depth, it is explained why C# generics doesn't support covariance and contra-variance. It is mainly for writing secure code. Now, C# 4.0 changed to support them. Would it bring chaos?
在 Jon Skeet 的C# in Depth一书中，解释了为什么 C# 泛型不支持协变和逆变。它主要用于编写安全代码。现在，C# 4.0 更改为支持它们。会带来混乱吗？
Anybody know the details about C# 4.0 can give some explanation?
有人知道有关 C# 4.0 的详细信息，可以给一些解释吗？
采纳答案by Jon Skeet
Variance will only be supported in a safeway - in fact, using the abilities that the CLR already has. So the examples I give in the book of trying to use a List<Banana>as a List<Fruit>(or whatever it was) still won't work - but a few other scenarios will.
仅以安全的方式支持差异- 实际上，使用 CLR 已有的功能。所以我在书中给出的尝试将 aList<Banana>用作 a List<Fruit>（或任何它是）的例子仍然不起作用 - 但其他一些场景会起作用。
Firstly, it will only be supported for interfaces and delegates.
首先，它只支持接口和委托。
Secondly, it requires the author of the interface/delegate to decorate the type parameters as in(for contravariance) or out(for covariance). The most obvious example is IEnumerable<T>which only ever lets you take values "out" of it - it doesn't let you add new ones. That will become IEnumerable<out T>. That doesn't hurt type safety at all, but lets you return an IEnumerable<string>from a method declared to return IEnumerable<object>for instance.
其次，它要求接口/委托的作者将类型参数修饰为in（对于逆变）或out（对于协方差）。最明显的例子是IEnumerable<T>它只允许你从它“取出”值——它不允许你添加新的值。那将成为IEnumerable<out T>. 这根本不会损害类型安全，但可以让您IEnumerable<string>从声明为 return 的方法返回一个IEnumerable<object>。
Contravariance is harder to give concrete examples for using interfaces, but it's easy with a delegate. Consider Action<T>- that just represents a method which takes a Tparameter. It would be nice to be able to convert seamlessly use an Action<object>as an Action<string>- any method which takes an objectparameter is going to be fine when it's presented with a stringinstead. Of course, C# 2 already has covariance and contravariance of delegates to some extent, but via an actual conversion from one delegate type to another (creating a new instance) - see P141-144 for examples. C# 4 will make this more generic, and (I believe) will avoid creating a new instance for the conversion. (It'll be a reference conversion instead.)
逆变很难给出使用接口的具体示例，但使用委托很容易。考虑Action<T>- 这仅代表一个接受T参数的方法。能够无缝地转换使用 an Action<object>as an会很好Action<string>- 任何采用object参数的方法在使用 a 时都可以string。当然，C# 2 在某种程度上已经具有委托的协变和逆变，但是通过从一种委托类型到另一种委托类型的实际转换（创建一个新实例） - 有关示例，请参见 P141-144。C# 4 将使这更通用，并且（我相信）将避免为转换创建新实例。（这将是一个参考转换。）
Hope this clears it up a bit - please let me know if it doesn't make sense!
希望这能让我明白一点 - 如果它没有意义，请告诉我！
回答by kemiller2002
Not that Jon hasn't already covered it, but here are some links to blogs and videos from Eric Lippert. He does a nice job of explaining it with examples.
并不是说 Jon 还没有涵盖它，但这里有一些指向 Eric Lippert 的博客和视频的链接。他用例子很好地解释了它。
https://blogs.msdn.microsoft.com/ericlippert/2007/10/16/covariance-and-contravariance-in-c-part-one/
https://blogs.msdn.microsoft.com/ericlippert/2007/10/16/covariance-and-contravariance-in-c-part-one/
The videos:
视频：
https://www.youtube.com/watch?v=3MQDrKbzvqU
https://www.youtube.com/watch?v=3MQDrKbzvqU
https://www.youtube.com/watch?v=XRIadQaBYlI
https://www.youtube.com/watch?v=XRIadQaBYlI
