为什么 C#/CLR 不支持方法覆盖协变/逆变？
有很多问题& 关于 C# 不允许方法返回（和参数）类型在覆盖时更改为兼容类型的限制的答案，但是为什么存在此限制，无论是在 C# 编译器中还是在 CLR 中？ 正如我所见，如果允许协变/逆变，那么没有什么可以破坏的，那么其背后的原因是什么？
对于扩大访问参数，可能会提出类似的问题 - 例如，用公共方法覆盖受保护的内部方法（Java 支持的东西，IIRC）
There are quite a few questions & answers about hacking around the limitation of C# not allowing method return (and argument) types to be changed to compatible types on overrides, but why does this limitation exist, either in the C# compiler or in the CLR? As I an see, there is nothing that could break if co/contra-variance was allowed, so what is the reasoning behind it?
A similar question could be asked for widening access parameters - eg overriding a protected internal method with a public method (something which Java supports, IIRC)
如果你对这篇内容有疑问，欢迎到本站社区发帖提问 参与讨论，获取更多帮助，或者扫码二维码加入 Web 技术交流群。
绑定邮箱获取回复消息
由于您还没有绑定你的真实邮箱，如果其他用户或者作者回复了您的评论，将不能在第一时间通知您！
发布评论
评论（5）
这个答案不是在谈论 C#，但它帮助我更好地理解问题，也许它会帮助其他人： 为什么没有参数对立 -覆盖的方差？
This answer is not talking about C#, but it helped me understand the issues better and maybe it will help others: Why is there no parameter contra-variance for overriding?
埃里克·利珀特（Eric Lippert）已经比我更好地回答了这个问题。
查看他的系列 C# 中的协变和逆变
和
如何实现C# 4.0 通用协方差 & 实现了逆变？
编辑：Eric 指出他没有谈论返回类型协变，但我决定保留此答案中的链接，因为这是一系列很酷的文章，如果查看，有人可能会发现它很有用上这个话题。
此功能已被请求，并且大约 5 年前 Microsoft 已回复道：“感谢您记录此信息。我们经常听到这个请求。我们会在下一个版本中考虑它。”
现在我将引用 Jon Skeet 的话，因为如果没有 Jon Skeet 的回答，这将不是 StackOverflow 上的正确答案。 协方差和 void 返回类型< /a>
Eric Lippert already answered this way better than I could.
Check out his series on Covariance and Contravariance in C#
and
How does C# 4.0 Generic Covariance & Contra-variance Implmeneted?
EDIT: Eric pointed out that he doesn't talk about return type convariance but I decided to keep the link in this answer because it is a cool series of articles and someone might find it useful if looking up this topic.
This feature has been requested and almost 5 years ago Microsoft has responded with "Thanks for logging this. We hear this request a lot. We'll consider it for the next release."
And now I'll quote Jon Skeet because it would not be a proper answer on StackOverflow without an answer by Jon Skeet. Covariance and void return types
引入返回值协方差的 Seams 没有 Java 和 C++ 所使用的本质缺点。 然而，引入形式参数的逆变确实会造成混乱。 我认为 C++ 中的这个答案 https://stackoverflow.com/a/3010614/1443505 对于 C# 也有效。
Seams introducing covariance of return value has no essential drawback as Java and C++ have used. However, there's real confusion cased by introducing contra-variance of formal parameter. I think this answer https://stackoverflow.com/a/3010614/1443505 in C++ is also valid for C#.
确实如此，你只需要等待 VS2010/.Net 4.0 即可。
It does, you just have to wait for VS2010/.Net 4.0.
为了扩展 Joel 的答案 - CLR 长期以来一直支持有限的方差，但 C# 编译器直到 4.0 才使用它们，并在通用接口和委托上添加了新的“in”和“out”修饰符。 原因很复杂，我解释起来会很混乱，但事情并不像看起来那么简单。
将“受保护的内部”方法重新制作为“公共”方法； 您可以通过方法隐藏来做到这一点：（
只要参数等也都是公开的）-有什么用吗？
To expand on Joel's answer - the CLR has supported limited variance for a long time, but the C# compiler doesn't use them until 4.0 with the new "in" and "out" modifiers on generic interfaces and delegates. The reasons are complicated, and I would get into a mess trying to explain, but it isn't as simple as it seems.
Re making a "protected internal" method into a "public" method; you can do this with method hiding:
(as long as the arguments etc are all also public) - any use?