C#中的案例声明块级别声明空间 [英] Case Statement Block Level Declaration Space in C#

问题描述

我是否有理由不将case语句中的块视为块级声明空间?

Is there a reason I am missing that a block within a case statement isn't considered a block level declaration space?

尝试时不断出现错误(变量已声明)

I keep getting an error (variable has already been declared) when I try

case x:
  var someVariable = 42;
break;
case y: 
   var someVariable = 40;
break;

但是我可以做

case x:
   try{var someVariable = 42;}catch{} 
break;
case y: 
    try{var someVariable = 40;}catch{}
break;

如果C#允许使用直通语句，那将是有道理的，但那没有意义，而且我想不出一种情况，您可以在case语句中声明变量并在该块之外使用它.

If C# allowed fall through statements, that would make sense, but it doesn't, and I can't think of a scenario where you can declare a variable in a case statement and use it outside of that block.

推荐答案

更新:此问题用作此博客文章的启发；看到它以获取更多详细信息.

UPDATE: This question was used as the inspiration for this blog post; see it for further details.

http://ericlippert.com/2009/08/13/four -switch-oddities/

感谢有趣的问题.

在其他各种答案中也有许多困惑和错误陈述，但没有一个可以真正解释为什么这是非法的.我将尽力而为.

There are a number of confusions and mis-statements in the various other answers, none of which actually explain why this is illegal. I shall attempt to be definitive.

首先，严格来说，"scope"是用于描述问题的错误词.巧合的是，我上周写了一篇博客文章，内容涉及对范围"的确切误用.它将在我的迭代器模块系列文章之后发布，该系列文章将在整个7月运行.

First off, to be strictly correct, "scope" is the wrong word to use to describe the problem. Coincidentally, I wrote a blog post last week about this exact mis-use of "scope"; that will be published after my series on iterator blocks, which will run throughout July.

正确使用的术语是"声明空间".声明空间是一个代码区域，在其中不能声明两个不同的事物具有相同的名称.此处描述的情况是以下事实的症状:尽管开关 block 定义了开关 section ，但没有定义声明空间.两个声明在同一个声明空间中并且具有相同的名称，它们是非法的.

The correct term to use is "declaration space". A declaration space is a region of code in which no two different things may be declared to have the same name. The scenario described here is symptomatic of the fact that a switch section does not define a declaration space, though a switch block does. Since the OP's two declarations are in the same declaration space and have the same name, they are illegal.

(是，切换块也定义了范围，但该事实与问题无关，因为问题与声明的合法性有关，而不与<标识符查询的strong>语义.)

(Yes, the switch block also defines a scope but that fact is not relevant to the question because the question is about the legality of a declaration, not the semantics of an identifier lookup.)

一个合理的问题是为什么这不合法?"一个合理的答案是嗯，为什么要这样"?您可以使用以下两种方法之一.这是合法的:

A reasonable question is "why is this not legal?" A reasonable answer is "well, why should it be"? You can have it one of two ways. Either this is legal:

switch(y)
{
case 1:  int x = 123; ... break;
case 2:  int x = 456; ... break;
}

或者这是合法的:

switch(y)
{
case 1:  int x = 123; ... break;
case 2:  x = 456; ... break;
}

但是您不能同时使用这两种方式. C#的设计师选择第二种方法似乎是更自然的方法.

but you can't have it both ways. The designers of C# chose the second way as seeming to be the more natural way to do it.

这项决定是在1999年7月7日做出的，距十年前还差.当天的注释中的注释非常简短，仅说明"一个开关箱不会创建自己的声明空间"，然后提供一些示例代码来显示有效和无效的代码.

This decision was made on July 7th, 1999, just shy of ten years ago. The comments in the notes from that day are extremely brief, simply stating "A switch-case does not create its own declaration space" and then giving some sample code that shows what works and what does not.

要想了解有关这一特定日子设计师的想法的更多信息，我不得不让很多人对十年前的想法有所误解，并让他们误解最终是一件微不足道的问题.我不会那样做.

To find out more about what was in the designers minds on this particular day, I'd have to bug a lot of people about what they were thinking ten years ago -- and bug them about what is ultimately a trivial issue; I'm not going to do that.

简而言之，没有特别令人信服的理由选择一种或另一种方式；两者都有优点.语言设计团队选择了一种方法，因为他们必须选择一种方法.他们选择的那一款对我来说似乎很合理.

In short, there is no particularly compelling reason to choose one way or the other; both have merits. The language design team chose one way because they had to pick one; the one they picked seems reasonable to me.

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