Powershell 中的空值合并 [英] Null coalescing in powershell

问题描述

powershell 中是否有空合并运算符?

我希望能够在 powershell 中执行这些 c# 命令:

var s = myval ??新价值"；var x = myval == null ?"" : 其他值;

解决方案

Powershell 7+

Powershell 7 在 Powershell 中引入了原生空值合并、空值条件赋值和三元运算符.

空合并

$null ??100 # 结果是 100评价"??(Expensive-Operation "Not Evaluated") # 这里的右边没有被评估

空条件赋值

$x = $null$x ??= 100 # $x 现在是 100$x ??= 200 # $x 仍然是 100

三元运算符

$true ?返回此值":未计算此表达式"$假?未计算此表达式":返回此值"

<小时>

以前的版本:

不需要 Powershell 社区扩展，您可以使用标准的 Powershell if 语句作为表达式:

variable = if (condition) { expr1 } else { expr2 }

所以要替换您的第一个 C# 表达式:

var s = myval ??新价值"；

成为以下之一(取决于偏好):

$s = if ($myval -eq $null) { "新值" } else { $myval }$s = if ($myval -ne $null) { $myval } else { "新值" }

或者取决于您可以使用的 $myval 可能包含的内容:

$s = if ($myval) { $myval } else { "新值" }

和第二个 C# 表达式以类似的方式映射:

var x = myval == null ?"" : 其他值;

变成

$x = if ($myval -eq $null) { "" } else { $otherval }

说句公道话，这些都不是很活泼，也远不如 C# 形式那么舒服.

您也可以考虑将其包装在一个非常简单的函数中以提高可读性:

function Coalesce($a, $b) { if ($a -ne $null) { $a } else { $b } }$s = Coalesce $myval 新值"

或者可能是，IfNull:

function IfNull($a, $b, $c) { if ($a -eq $null) { $b } else { $c } }$s = IfNull $myval "新值" $myval$x = IfNull $myval "" $otherval

如您所见，一个非常简单的函数可以为您提供相当多的语法自由.

更新:混合中需要考虑的一个额外选项是更通用的 IsTrue 函数:

function IfTrue($a, $b, $c) { if ($a) { $b } else { $c } }$x = IfTrue ($myval -eq $null) "" $otherval

然后结合 Powershell 声明看起来有点像运算符的别名的能力，你最终得到:

新别名??"合并$s = ??$myval "新值"新别名 "?:" IfTrue$ans = ?: ($q -eq "生命的意义") 42 $otherval

显然，这不会符合每个人的口味，但可能是您正在寻找的.

正如 Thomas 所指出的，C# 版本与上述版本之间的另一个细微差别是 C# 执行参数的短路，但涉及函数/别名的 Powershell 版本将始终评估所有参数.如果这是一个问题，请使用 if 表达式形式.

Is there a null coalescing operator in powershell?

I'd like to be able to do these c# commands in powershell:

var s = myval ?? "new value";
var x = myval == null ? "" : otherval;

解决方案

Powershell 7+

Powershell 7 introduces native null coalescing, null conditional assignment, and ternary operators in Powershell.

Null Coalescing

$null ?? 100    # Result is 100

"Evaluated" ?? (Expensive-Operation "Not Evaluated")    # Right side here is not evaluated

Null Conditional Assignment

$x = $null
$x ??= 100    # $x is now 100
$x ??= 200    # $x remains 100

Ternary Operator

$true  ? "this value returned" : "this expression not evaluated"
$false ? "this expression not evaluated" : "this value returned"

---

Previous Versions:

No need for the Powershell Community Extensions, you can use the standard Powershell if statements as an expression:

variable = if (condition) { expr1 } else { expr2 }

So to the replacements for your first C# expression of:

var s = myval ?? "new value";

becomes one of the following (depending on preference):

$s = if ($myval -eq $null) { "new value" } else { $myval }
$s = if ($myval -ne $null) { $myval } else { "new value" }

or depending on what $myval might contain you could use:

$s = if ($myval) { $myval } else { "new value" }

and the second C# expression maps in a similar way:

var x = myval == null ? "" : otherval;

becomes

$x = if ($myval -eq $null) { "" } else { $otherval }

Now to be fair, these aren't very snappy, and nowhere near as comfortable to use as the C# forms.

You might also consider wrapping it in a very simple function to make things more readable:

function Coalesce($a, $b) { if ($a -ne $null) { $a } else { $b } }

$s = Coalesce $myval "new value"

or possibly as, IfNull:

function IfNull($a, $b, $c) { if ($a -eq $null) { $b } else { $c } }

$s = IfNull $myval "new value" $myval
$x = IfNull $myval "" $otherval

As you can see a very simple function can give you quite a bit of freedom of syntax.

UPDATE: One extra option to consider in the mix is a more generic IsTrue function:

function IfTrue($a, $b, $c) { if ($a) { $b } else { $c } }

$x = IfTrue ($myval -eq $null) "" $otherval

Then combine that is Powershell's ability to declare aliases that look a bit like operators, you end up with:

New-Alias "??" Coalesce

$s = ?? $myval "new value"

New-Alias "?:" IfTrue

$ans = ?: ($q -eq "meaning of life") 42 $otherval

Clearly this isn't going to be to everyone's taste, but may be what you're looking for.

As Thomas notes, one other subtle difference between the C# version and the above is that C# performs short-circuiting of the arguments, but the Powershell versions involving functions/aliases will always evaluate all arguments. If this is a problem, use the if expression form.

这篇关于Powershell 中的空值合并的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！