C# switch 语句中的变量声明
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Variable declaration in a C# switch statement
提问by Jeremy
Why is it that in a C# switch statement, for a variable used in multiple cases, you only declare it in the first case?
为什么在C#的switch语句中，对于一个在多种情况下使用的变量，你只在第一种情况下声明？
For example, the following throws the error "A local variable named 'variable' is already defined in this scope".
例如，下面的代码抛出错误“一个名为‘variable’的局部变量已经在这个作用域中定义”。
switch (Type)
{
case Type.A:
string variable = "x";
break;
case Type.B:
string variable = "y";
break;
}
However, per the logic, the initial declaration should not be hit if the type is Type.B. Do all variables within a switch statement exist in a single scope, and are they created/allocated before any logic is processed?
但是，根据逻辑，如果类型为 ，则不应命中初始声明Type.B。switch 语句中的所有变量是否都存在于单个作用域中，它们是否在处理任何逻辑之前创建/分配？
采纳答案by Mitchel Sellers
I believe it has to do with the overall scope of the variable, it is a block level scope that is defined at the switch level.
我相信它与变量的整体范围有关，它是在开关级别定义的块级别范围。
Personally if you are setting a value to something inside a switch in your example for it to really be of any benefit, you would want to declare it outside the switch anyway.
就个人而言，如果您在示例中为开关内的某些内容设置值以使其真正有任何好处，则无论如何您都希望在开关外声明它。
回答by Jon Skeet
Yes, the scope is the entire switch block - unfortunately, IMO. You can always add braces within a single case, however, to create a smaller scope. As for whether they're created/allocated - the stack frame has enough space for all the local variables in a method (leaving aside the complexities of captured variables). It's not like that space is allocated during the method's execution.
是的，范围是整个开关块 - 不幸的是，IMO。但是，您始终可以在单个案例中添加大括号以创建更小的范围。至于它们是否被创建/分配 - 堆栈帧有足够的空间用于方法中的所有局部变量（撇开捕获变量的复杂性）。这不像在方法执行期间分配空间。
回答by itsmatt
Because their scope is at the switch block. The C# Language Specificationstates the following:
因为它们的作用域在 switch 块上。在C＃语言规范规定如下：
The scope of a local variable or constant declared in a switch block is the switch block.
在 switch 块中声明的局部变量或常量的作用域是 switch 块。
回答by James Curran
The initialization takes place in the case, but the declaration is effectively done at the top of the scope. (Psuedo-code)
初始化发生在 case 中，但声明实际上是在作用域的顶部完成的。（伪代码）
switch (Type)
{
string variable;
case Type.A:
variable = "x";
break;
case Type.B:
variable = "y";
break;
}
回答by jezell
The variables do share scope in the C# compiler. However, scope doesn't exist in the same way in CIL. As for actual creation / initialization... the .NET memory model lets the compiler move reads / writes a bit as long as simple rules are followed unless the variable is marked as volatile.
这些变量在 C# 编译器中共享作用域。但是，作用域在CIL中并不以相同的方式存在。至于实际的创建/初始化... .NET 内存模型允许编译器稍微移动读取/写入，只要遵循简单的规则，除非变量被标记为volatile。
回答by Michael Burr
If you want a variable scoped to a particular case, simply enclose the case in its own block:
如果你想要一个作用域为特定情况的变量，只需将该情况包含在它自己的块中：
switch (Type)
{
case Type.A:
{
string variable = "x";
/* Do other stuff with variable */
}
break;
case Type.B:
{
string variable = "y";
/* Do other stuff with variable */
}
break;
}
回答by percebus
"In My Daaaaays..."
“在我的天啊……”
swicthis a really primitive procedural implementation that has been around since the ages of Citself (even before C++).
swicth是一个非常原始的过程实现，它自其时代以来就已经存在C（甚至在 之前C++）。
The whole switchis a blockthat serves as a scope-contained GOTO:(hence the :in each case). If you took some assembler classes, that might seem familiar.
整体switch是一个块，用作范围包含GOTO:（因此:在 each case）。如果您参加了一些汇编课程，那可能看起来很熟悉。
That is why switchuse is most helpful when combining with Enums and not using breakin every single caselike
这就是为什么switchuse 在与Enums结合而不是break在每个caselike 中使用时最有用
switch(mood)
{
case Mood.BORED:
case Mood.HAPPY:
drink(oBeer) // will drink if bored OR happy
break;
case Mood.SAD: // unnecessary but proofs a concept
default:
drink(oCoffee)
break;
}
