c# - 变量破坏范围

Question

这怎么会给出一个 's' does not exist in the current context 错误(正如预期的那样):

public static void Main()
{
    foreach(var i in new[]{1, 2, 3}) {
        int s = i;
    }

    Console.WriteLine(s);
}

( ideone )

但这会产生一个 's' cannot be redeclared 错误？

public static void Main()
{
    foreach(var i in new[]{1, 2, 3}) {
        int s = i;
    }

    int s = 4;
}

( ideone )

第一个错误告诉我 s 不存在于 foreach 之外，这是有道理的，但第二个错误则不然。为什么(以及如何!？)我需要从子作用域访问变量吗？

Answer 1

The first error tells me that s doesn't exist outside of the foreach, which makes sense

确实 - 这是正确的。

but the second error says otherwise.

不，它没有。它告诉您不能声明第一个(嵌套)变量s，因为第二个 已经在范围内。您无法在声明之前访问它，但它在整个 block 的范围内。

来自 C# 5 规范，第 3.7 节:

• The scope of a local variable declared in a local-variable-declaration (§8.5.1) is the block in which the declaration occurs.

所以是的，它基本上向上延伸到封闭的 {。

然后从 8.5.1 节开始:

The scope of a local variable declared in a local-variable-declaration is the block in which the declaration occurs. It is an error to refer to a local variable in a textual position that precedes the local-variable-declarator of the local variable. Within the scope of a local variable, it is a compile-time error to declare another local variable or constant with the same name.

最后一部分(强调我的)是您收到错误的原因。

Why (and how!?) would I ever need to access a variable from a child scope?

不确定您在这里的意思，但基本规则是让您更难编写难以阅读或脆弱的代码。这意味着向上或向下移动变量声明(但仍在同一 block 中，在同一嵌套级别)产生有效但具有不同含义的代码的地方更少。