在泛型集合多态类型参数

问题描述

为什么C＃编译器不允许多态型（T）的泛型集合参数（即列表[T]）？

Why does the C# compiler not allow polymorphic type (T) parameters in generic collections (ie, List[T]) ?

以一流的'A'和' b'例如，在'b'是的子类A

Take class 'A' and 'B' for example, where 'B' is a subclass of 'A'

class A { }
class B : A { }

和考虑一个函数，类型的列表'A'

and consider a function that takes a list of type 'A'

$

void f(List<A> aL) { }

that gets called with a list of type 'B'

List<B> bL = new List<B>();

f(bL);

给出以下错误

The following error is given

ERROR: cannot convert from List<B> to List<A>

什么语义规则被侵犯？

What semantic rule is being violated ?

也有一个优雅的意思，为此，除了通过循环和铸造每个元素（我要加点糖请）？谢谢

Also is there an "elegant" mean to this end, aside from looping through and casting each element (I want some sugar please) ? Thanks.

推荐答案

列表< B> 根本就不是一个亚型列表与LT; A> 。 （我从来没有肯定什么协变，什么逆变就是在这种背景下，所以我会坚持以子类型）。在那里你做到这一点考虑这样的情况：

List<B> simply is not a subtype of List<A>. (I'm never sure about what "covariant" and what "contravariant" is in this context so I'll stick with "subtype".) Consider the case where you do this:

void Fun(List<A> aa) {
    aa(new A());
}

var bb = new List<B>();
Fun(bb); // whoopsie

如果你想要做的是什么允许将有可能增加一个 A 到 b ■哪些显然不是类型安全的列表。

If what you want to do was allowed it would be possible to add an A to a list of Bs which is clearly not type-safe.

现在，显然有可能的阅读的从列表安全，这也是为什么C＃让你创建的协（即只读）接口 - 这让编译器知道这是不可能通过他们造成这种损坏。如果你只需要读访问，对集合，通常一个是的IEnumerable< T> ，所以你的情况，你可能只是使该方法：

Now, clearly it's possible to read elements from the list safely, which is why C# lets you create covariant (i.e. "read-only") interfaces - which let the compiler know it's not possible to cause this sort of corruption through them. If you only need read access, for collections, the usual one is IEnumerable<T>, so in your case you might just make the method:

$ b

void Fun(IEnumerable<A> aa) { ... }

and use the Enumerable methods - most should be optimised if the underlying type is List.

不幸的是，由于C＃泛型的东西是如何工作的，的类的不能变可言，只有接口。而据我所知，所有的集合接口比富裕的IEnumerable< T> 是读写。你可以在技术上使自己的协变封装接口，仅公开您需要的读操作。

Unfortunately, because of how the C# generics stuff works, classes can't be variant at all, only interfaces. And as far as I know, all the collection interfaces "richer" than IEnumerable<T> are "read-write". You could technically make your own covariant wrapper interface that only exposes the read operations you want.

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