为什么C#/ CLR不支持方法重写CO /禁忌变化? [英] Why does C#/CLR not support method override co/contra-variance?
问题描述
有相当多的问题和放大器;关于黑客周围的C#不允许方法返回(及参数)类型更改为兼容的类型上覆盖的限制,但答案为什么没有这种限制存在,无论是在C#编译器或CLR?正如我在看,没有什么可以打破,如果CO /禁忌变化是允许的,那么什么是其背后的原因何在呢?
有一个类似的问题可能会被要求扩大接入参数 - 比如覆盖一个受保护的内部方法与公共法(一些东西,Java支持,IIRC)
埃里克利珀已经回答了这个办法比我所能。
查看他的系列<一href="http://blogs.msdn.com/ericlippert/archive/2007/10/16/covariance-and-contravariance-in-c-part-one.aspx"相对=nofollow>协变和逆变的C#
和
<一个href="http://stackoverflow.com/questions/245607/how-does-c-40-generic-covariance-contra-variance-implmeneted">How做C#4.0泛型协方差和放大器;魂斗罗方差Implmeneted?
编辑:埃里克指出,他没有谈及返回类型协方差,但我决定继续在这个答案的链接,因为它是一个很酷的系列文章,有人可能会发现它有用的,如果找了这个话题。 P>
此功能已要求以及近5年前微软的回答是:谢谢你的记录本。我们听到这个要求很多,我们会考虑它的下一个版本。
而现在我还要举乔恩斯基特,因为它不会在计算器一个合适的回答,而不由乔恩飞碟双向的答案。 协方差和无效的返回类型一>
我强烈怀疑,答案 在于CLR执行 而不是在任何深层语义 原因 - CLR可能需要 知道能不能有将 是一个返回值,以做 与堆栈合适的事情。 即便如此,似乎有点可惜的,在 术语的优雅。我不能说我已经 曾经觉得需要这在现实 生活中,这将是相当容易 假(最多四个参数)的 .NET 3.5只是写一个转换器 从
函数功能:其中,X&GT;
到动作&其中,X&GT;
,函数功能:其中,X,Y&GT;
以动作&LT; X,Y&GT;
等,niggles位 虽然:)
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)
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
I strongly suspect that the answer lies in the implementation of the CLR rather than in any deep semantic reason - the CLR probably needs to know whether or not there's going to be a return value, in order to do appropriate things with the stack. Even so, it seems a bit of a pity, in terms of elegance. I can't say I've ever felt the need for this in real life, and it would be reasonably easy to fake (for up to four parameters) in .NET 3.5 just by writing a converter from
Func<X>
toAction<X>
,Func<X,Y>
toAction<X,Y>
etc. It niggles a bit though :)
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