变量更改值无需重新分配 [英] Variable changes value without re-assignment

问题描述

我有一个变量，它通过我更新不同变量的值来改变它的值.

我正在尝试创建自己的对象，并尝试使用我在网上找到的一些代码，但我偶然发现了这个问题，无法理解这是如何发生的.

准备工作:

$a = "" |select First,Last #This 只是创建一个具有两列的自定义对象.$b = @() #这只是一个空数组，稍后用'$A'的值填充$a.first = "阿尔法"$a.last = "Bravo"$b += $a$a.first = "查理"$a.last = "三角洲"$b += $a

应该发生什么:

<前>首尾----- ----阿尔法布拉沃查理三角洲

但实际结果是:

<前>首尾----- ----查理三角洲查理三角洲

这里出了什么问题?

解决方案

正如 Lee_Dailey 指出的那样，你'将 references 添加到存储在 $a 中的自定义对象 ([pscustomobject]^[1]) 实例到数组$b(假设 [pscustomobject] 是引用类型)，并且您正在更新相同的自定义对象，因此您最终与:

• 2 个指向同一个对象的数组元素...
• ...其属性值是您最后分配的值.

<小时>

最直接的解决方法是在分配新的属性值之前创建 $a 的copy，这可以通过 .psobject.Copy()，它创建了一个 - 浅 - 克隆:

<代码>...# 创建 $a 的副本以创建下一个数组元素.$a = $a.psobject.Copy()$a.first = "查理"$a.last = "三角洲"$b += $a

当然，您可以避免这个问题并使用自定义对象文字(PSv3+)代替，它每次都会创建一个新实例:

$b = @()$b += [pscustomobject] @{第一个 = "阿尔法"last = "Bravo"}$b += [pscustomobject] @{第一个 = "查理"最后=德尔塔"}

顺便说一句:使用 += 增长数组效率低下，因为每次都必须在幕后创建一个新数组；这可能不会在只有少数添加/迭代时引起注意，但如果数量较多，最好使用列表数据类型，例如 [System.Collections.Generic.List[object]]::>

$b = 新对象 System.Collections.Generic.List[object]$b.Add([pscustomobject] @{第一个 = "阿尔法"last = "Bravo"})$b.Add([pscustomobject] @{第一个 = "查理"最后=德尔塔"})

<小时>

[1] PowerShell 中的自定义对象:

[pscustomobject](实际上与 [psobject] 相同)是 PowerShell 的属性包"类型，它允许您ad hoc 不需要提前声明不同的 .NET 类型；从 .NET 的角度来看，给定的自定义对象的类型始终是 System.Management.Automation.PSCustomObject，但具体的动态附加属性可能因设计而异.

Select-Object (select) 也输出 [pscustomobject] 实例，尽管它们通过 PowerShell 的 ETS(扩展类型系统)，作为它们的主要类型名称:

PS>("" | 选择第一个，最后一个).pstypenamesSelected.System.String #自定义类型名称以反映*input*的类型System.Management.Automation.PSCustomObject # 真正的类型名称System.Object # 基类型的名称

上述创建自定义对象的方法已过时，但是，PSv3+ 支持直接构造的文字 [pscustomobject] @{ ... } 语法，它具有添加了能够将属性作为同一语句的一部分初始化并且比Select-Object(以及New-对象):

# PSv3+ 相当于上面的#(自定义的Selected.System.String"类型名称除外)，# 允许你也初始化属性.[pscustomobject] @{第一个 = $null最后= $null}

^{注意:在 PSv2 中，您可以使用 New-Object PSCustomObject -Property @{ ... } 达到类似的效果，但附加属性的顺序通常不会反映定义顺序.}

请注意，您始终也可以在 PowerShell 中实例化常规 .NET 类型，使用 New-Object cmdlet 或者，在 PSv5+ 中，或者使用语法 []::new([...]).您使用相同的方法来实例化 PSv5+ 自定义类 用 class 关键字声明.

I have a variable that changes it's values by me updating the values from a different variable.

I'm trying to create my own object and was experimenting with some code I found online and I stumbled on this issue and can't wrap my head on how this happens.

Preparations:

$a = "" | select First,Last #This just creates a custom object with two columns.
$b = @() #This is just an empty array to fill later on with the values of '$A'

$a.first = "Alpha"
$a.last = "Bravo"
$b += $a

$a.first = "Charlie"
$a.last = "Delta"
$b += $a

What is supposed to happen:

First   Last
-----   ----
Alpha   Bravo
Charlie Delta

But the actual results are:

First   Last
-----   ----
Charlie Delta
Charlie Delta

What is wrong here?

解决方案

As Lee_Dailey points out, you're adding references to the custom object ([pscustomobject]^[1]) instance stored in $a to array $b (given that [pscustomobject] is a reference type), and you're updating the same custom object, so that you ended up with:

• 2 array elements that point to the very same object...
• ... whose property values are the last ones you assigned.

---

The most immediate fix is to create a copy of $a before assigning new property values, which can be done with .psobject.Copy(), which creates a - shallow - clone:

...
# Create a copy of $a to create the next array element.
$a = $a.psobject.Copy()

$a.first = "Charlie"
$a.last = "Delta"
$b += $a

Of course, you could avoid the problem and use custom-object literals (PSv3+) instead, which creates a new instance every time:

$b = @()

$b += [pscustomobject] @{ 
  first = "Alpha"
  last = "Bravo"
}

$b += [pscustomobject] @{ 
  first = "Charlie"
  last = "Delta"
}

As an aside: growing arrays with += is inefficient, because a new array must be created behind the scenes every time; this may not be noticeable with only a few additions / iterations, but with a larger number it's better to use a list data type such as [System.Collections.Generic.List[object]]:

$b = New-Object System.Collections.Generic.List[object]

$b.Add([pscustomobject] @{ 
  first = "Alpha"
  last = "Bravo"
})

$b.Add([pscustomobject] @{ 
  first = "Charlie"
  last = "Delta"
})

---

[1] Custom objects in PowerShell:

[pscustomobject] (which is effectively the same as [psobject]) is PowerShell's "property bag" type that allows you to construct objects ad hoc without requiring a distinct .NET type declared ahead of time; from the perspective of .NET, a given custom object's type is always System.Management.Automation.PSCustomObject, though the specific, dynamically attached properties can differ, by design.

Select-Object (select) too outputs [pscustomobject] instances, though they report a custom type name, via PowerShell's ETS (Extended Type System), as their primary type name:

PS> ("" | select First,Last).pstypenames
Selected.System.String  # custom type name to reflect the type of the *input*
System.Management.Automation.PSCustomObject # the true type name
System.Object # the name of the base type

The above method of creating custom objects is obsolete, however, and PSv3+ supports literal [pscustomobject] @{ ... } syntax for direct construction, which has the added advantages of being able to initialize the properties as part of the same statement and being faster than Select-Object (and also New-Object):

# PSv3+ equivalent of the above 
# (except for the custom 'Selected.System.String' type name),
# allowing you to also initialize the properties.
[pscustomobject] @{
   First = $null  
   Last = $null 
}

^{Note: In PSv2 you could use New-Object PSCustomObject -Property @{ ... } to similar effect, but the order in which the properties are attached will typically not reflect the definition order.}

Note that you can always instantiate regular .NET types in PowerShell as well, with the New-Object cmdlet or, in PSv5+, alternatively with syntax [<type>]::new([...]). You use the same methods to instantiate PSv5+ custom classes declared with the class keyword.

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