PowerShell - “写输出"vs “返回"在函数中 [英] PowerShell - "Write-Output" vs "return" in functions
问题描述
我已经使用 PowerShell 多年了，我以为我可以处理它的一些更古怪"的行为，但我遇到了一个我无法理解的问题..
I've been using PowerShell for a number of years, and I thought I had a handle on some of its more 'eccentric' behaviour, but I've hit an issue I can't make head nor tail of...
我一直使用return"来从函数返回值，但最近我想我应该看看 Write-Output 作为替代方法.然而，PowerShell 是 PowerShell，我发现了一些似乎没有意义的东西(至少对我来说):
I've always used "return" to return values from functions, but recently I thought I'd have a look at Write-Output as an alternative. However, PowerShell being PowerShell, I've found something that doesn't seem to make sense (to me, at least):
function Invoke-X{ write-output @{ "aaa" = "bbb" } };
function Invoke-Y{ return @{ "aaa" = "bbb" } };
$x = Invoke-X;
$y = Invoke-Y;
write-host $x.GetType().FullName
write-host $y.GetType().FullName
write-host ($x -is [hashtable])
write-host ($y -is [hashtable])
write-host ($x -is [pscustomobject])
write-host ($y -is [pscustomobject])
输出:
System.Collections.Hashtable
System.Collections.Hashtable
True
True
True
False
$x 和 $y(或写输出"和返回")之间的区别是什么，这意味着它们都是哈希表，但其中只有一个-是"pscustomobject?是否有一种通用的方法可以确定与代码的差异，除了明显检查变量中的每个哈希表是否也是 pscustomobject)?
What is the difference between $x and $y (or 'write-output' and 'return') that means they're both hashtables, but only one of them '-is' a pscustomobject? And is there a generalised way I can determine the difference from code, other than obviously checking whether every hashtable I have in a variable is also a pscustomobject)?
我的 $PSVersionTable 看起来像这样，以防此行为特定于特定版本的 PowerShell:
My $PSVersionTable looks like this, in case this behaviour is specific to a particular version of PowerShell:
Name Value
---- -----
PSVersion 5.1.16299.492
PSEdition Desktop
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
BuildVersion 10.0.16299.492
CLRVersion 4.0.30319.42000
WSManStackVersion 3.0
PSRemotingProtocolVersion 2.3
SerializationVersion 1.1.0.1
干杯，
M
推荐答案
return 和 [pscustomobject] 在这里，从某种意义上说.
return and [pscustomobject] are red herrings here, in a way.
归结为:
隐式表达式输出与cmdlet产生的输出；使用
return(不使用 cmdlet 调用)属于前一类，使用Write-Output属于后者.
Implicit expression output vs. cmdlet-produced output; using
return(without a cmdlet call) falls into the former category, usingWrite-Outputinto the latter.
输出对象被包裹 - 大部分是不可见的 - [psobject] 实例仅在 cmdlet 生成的输出中.
Output objects getting wrapped in - mostly invisible - [psobject] instances only in cmdlet-produced output.
# Expression output: NO [psobject] wrapper:
@{ "aaa" = "bbb" } -is [psobject] # -> $False
# Cmdlet-produced output: [psobject]-wrapped
(Write-Output @{ "aaa" = "bbb" }) -is [psobject] # -> $True
请注意 - 令人惊讶的是 - [pscustomobject] 与 [psobject] 相同:它们都引用类型 [System.Management.Automation.PSObject]，这是 PowerShell 在幕后使用的通常不可见的辅助类型.
(为了增加混淆， 有一个单独的 [System.Management.Automation.PSCustomObject] 类型.)
Note that - surprisingly - [pscustomobject] is the same as [psobject]: they both refer to type [System.Management.Automation.PSObject], which is the normally invisible helper type that PowerShell uses behind the scenes.
(To add to the confusion, there is a separate [System.Management.Automation.PSCustomObject] type.)
在大多数情况下，这个额外的 [psobject] 包装器是良性的 - 它的行为大多与直接包装的对象一样 - 但在某些情况下，它会导致微妙的不同行为(见下文).
For the most part, this extra [psobject] wrapper is benign - it mostly behaves as the wrapped object would directly - but there are instances where it causes subtly different behavior (see below).
是否有一种通用的方法可以确定与代码的差异，除了明显检查变量中的每个哈希表是否也是 pscustomobject
And is there a generalised way I can determine the difference from code, other than obviously checking whether every hashtable I have in a variable is also a pscustomobject
请注意，哈希表不是一个 PS 自定义对象 - 它只出现在 - 任何 - [psobject] - 由于 [pscustomobject] 包装的对象] 与 [psobject] 相同.
Note that a hashtable is not a PS custom object - it only appears that way for - any - [psobject]-wrapped object due to [pscustomobject] being the same as [psobject].
检测真正的 PS 自定义对象 - 使用 [pscustomobject] @{ ... } 或 New-Object PSCustomObject/New-Object PSObject<创建/code> 或由 cmdlet 生成，例如 Select-Object 和 Import-Csv - 使用:
To detect a true PS custom object - created with [pscustomobject] @{ ... } or New-Object PSCustomObject / New-Object PSObject or produced by cmdlets such as Select-Object and Import-Csv - use:
$obj -is [System.Management.Automation.PSCustomObject] # NOT just [pscustomobject]!
请注意，从 Windows PowerShell v5.1/PowerShell Core v6.1.0 开始，将相关的 -as 运算符与真正的 PS 自定义对象一起使用已被破坏 - 见下文.
Note that using the related -as operator with a true PS custom object is broken as of Windows PowerShell v5.1 / PowerShell Core v6.1.0 - see below.
举个例子，额外的 [psobject] 包装器是良性的，你仍然可以直接测试包装对象的类型:
As an example of a situation where the extra [psobject] wrapper is benign, you can still test even a wrapped object for its type directly:
(Write-Output @{ "aaa" = "bbb" }) -is [hashtable] # $True
也就是说，尽管有包装器，-is 仍然识别 wrapped 类型.因此，有些矛盾的是，both -is [psobject] 和 -is [hashtable] 返回 $True 在这个案例，即使这些类型无关.
That is, despite the wrapper, -is still recognizes the wrapped type.
Therefore, somewhat paradoxically, both -is [psobject] and -is [hashtable] return $True in this case, even though these types are unrelated.
这些差异没有充分的理由，我认为它们是有漏洞的抽象(实现):不小心从窗帘后面偷看的内部结构.
There is no good reason for these discrepancies and they strike me as leaky abstractions (implementations): internal constructs accidentally peeking from behind the curtain.
以下 GitHub 问题讨论了这些行为:
The following GitHub issues discuss these behaviors:
Objects are situationally invisibly
[psobject]-wrapped, sometimes causing unexpected behavior.
为什么 [pscustomobject] 与 相同>[psobject]，即使存在不同的 [System.Management.Automation.PSCustomObject] 类型?
Why is [pscustomobject] the same as [psobject], even though a distinct [System.Management.Automation.PSCustomObject] type exists?
是否应将所有 [psobject] cmdlet 参数更改为[对象]?
Should all [psobject] cmdlet parameters be changed to [object]?
-as 运算符无法识别 System.Management.Automation.PSCustomObject 实例等.
The -as operator does not recognize System.Management.Automation.PSCustomObject instances as such.
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