引用全局变量时是否可以选择使用 $global: 范围前缀? [英] Is it optional to use the $global: scope prefix when referencing a global variable?

问题描述

如果我使用 $global:MyVariable = "123" 声明并初始化一个 PowerShell 变量，那么我是否需要使用 $global: 任何使用该变量的地方，还是可以只使用 $MyVariable?

If I declare and initialize a PowerShell variable with $global:MyVariable = "123" do I then need to use $global: anywhere I use the variable or can I just use $MyVariable?

推荐答案

• 原则上你可以只使用$MyVariable来引用一个全局变量的值，但您可能会看到不同的变量值，视情况而定.

  • In principle you can use just $MyVariable to reference the value of a global variable by that name, but you may see a different variable's value, situationally.

    具体来说，如果任何介入的祖先(父)范围甚至调用范围本身也创建了一个$MyVariable 变量(通常通过简单的赋值隐式发生；例如，$MyVariable = ...)，你会看到那个变量的值.

    Specifically, if any intervening ancestral (parent) scope or even the calling scope itself has also created a $MyVariable variable (which typically happens implicitly by simple assigmnent; e.g., $MyVariable = ...), you will see that variable's value instead.

    如果您确实使用了全局范围说明符，即:$global:MyVariable(或 Get-Variable -ValueOnly -Scope Global MyVariable)^[1]

    You're only guaranteed to see the global value if you do use the global scope specifier, namely: $global:MyVariable (or Get-Variable -ValueOnly -Scope Global MyVariable)^[1]

    默认情况下，变量在所有后代(子)范围内可见，但不能直接修改.

    By default, variables are visible, but not directly modifiable in all descendant (child) scopes.

    • 因此您必须使用$global:MyVariable/Set-Variable -Scope Global 以便修改(设置)一个全局变量；如果没有 $global:，您将隐式创建一个同名的 local 变量.
    • 有关 PowerShell 范围规则的更多信息，请参阅此答案的最后一部分.
    • Therefore you must use $global:MyVariable / Set-Variable -Scope Global in order to modify (set) a global variable; without $global: you'll implicitly create a local variable by the same name.
    • For more on PowerShell's scoping rules, see the last section of this answer.

    [1] 在实际场景中，即使是 PowerShell modules 中的代码也会看到带有作用域说明符 $global: 的全局变量，以及全局变量的隐式可见性总是适用.

    [1] In real-world scenarios, even code in PowerShell modules sees global variables with scope specifier $global:, and the implicit visibility of global variables always applies.

    对于内存模块，有一种方法可以让 $global/-Scope global 引用一个不同的范围，即模块自己的顶级作用域，但该技术晦涩难懂，没有记录，其实际效用未知.
    如果您想了解更多信息，请继续阅读.

    For in-memory modules, there is a way to make $global / -Scope global refer to a different scope, namely the module's own top-level scope, but the technique is obscure, not documented, and its real-world utility is unknown.
    Read on, if you want to know more.

    可选阅读:创建一个将自己的顶级范围视为全局范围的内存模块:

    Optional reading: Creating an in-memory module that sees its own top-level scope as the global scope:

    PetSerAl 发现了一种鲜为人知的方法来创建内存模块，使其能够看到$global:/-Scope global 作为其自己的顶级范围而不是真正的全局范围 - 奇怪的是，不是 使用显式范围引用使代码仍然可以看到(非阴影)全局变量:

    PetSerAl has discovered a little-known way to create an in-memory module in a manner that makes it see $global: / -Scope global as its own top-level scope rather than the true global scope - curiously, not using an explicit scope reference makes the code still see (non-shadowed) global variables:

    $global:MyVariable = 42 # Create a true global variable.
    
    # Create an in-memory module for which its *own top-level scope*
    # becomes the "global" scope, by virtue of passing $false to the
    # [psmoduleinfo] constructor:
    & ([psmoduleinfo]::new($false)) {
      @"
       '$global:MyVariable',
       '$(Get-Variable -ValueOnly -Scope global MyVariable)',
       '$MyVariable'
    "@
    }
    

    产量:

    Get-Variable : Cannot find a variable with the name 'MyVariable'.
    # ...
    
       '',  # $global:MyVariable didn't find the variable
       '',  # Neither did Get-Variable -Scope Global (see error above)
       '42' # OK - implicit visibility of true global variables
    

    请注意，New-Module 和 Import-Module(对于 persisted 模块)都没有提供此功能.

    Note that neither New-Module nor Import-Module (for persisted modules) offer this functionality.

    晦涩的&<模块信息>{ ... } 上面用于在模块范围内调用脚本块的技术在 这篇优秀的博文 作者:Patrick Meinecke.

    The obscure & <module-info> { ... } technique used above for invoking a script block in a module's scope is explained in this excellent blog post by Patrick Meinecke.

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