PowerShell 字符串默认参数值未按预期工作 [英] PowerShell string default parameter value does not work as expected

问题描述

#Requires -Version 2.0

[CmdletBinding()]
Param(
  [Parameter()] [string] $MyParam = $null
)

if($MyParam -eq $null) {
  Write-Host 'works'
} else {
  Write-Host 'does not work'
}

输出不起作用"=> 看起来字符串从 null 隐式转换为空字符串?为什么?以及如何测试字符串是空的还是真的 $null?这应该是两个不同的值！

Outputs "does not work" => looks like strings are converted from null to empty string implicitly? Why? And how to test if a string is empty or really $null? This should be two different values!

推荐答案

好的，找到答案@https://www.codykonior.com/2013/10/17/checking-for-null-in-powershell/

假设:

Param(
  [string] $stringParam = $null
)

并且没有指定参数(使用默认值):

And the parameter was not specified (is using default value):

# will NOT work
if ($null -eq $stringParam)
{
}

# WILL work:
if ($stringParam -eq "" -and $stringParam -eq [String]::Empty)
{
}

或者，您可以指定一个特殊的空类型:

Alternatively, you can specify a special null type:

Param(
  [string] $stringParam = [System.Management.Automation.Language.NullString]::Value
)

在这种情况下 $null -eq $stringParam 将按预期工作.

In which case the $null -eq $stringParam will work as expected.

奇怪！

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