PowerShell string default parameter value does not

问题:

#Requires -Version 2.0

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

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

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!

回答1:

Okay, found the answer @ https://www.codykonior.com/2013/10/17/checking-for-null-in-powershell/

Assuming:

Param(
  [string] $stringParam = $null
)

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)
{
}

Alternatively, you can specify a special null type:

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

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

Weird!

回答2:

You will need to use the AllowNull attribute if you want to allow $null for string parameters:

[CmdletBinding()]
Param (
    [Parameter()] 
    [AllowNull()]
    [string] $MyParam
)

And note that you should use $null on the left-hand side of the comparison:

if ($null -eq $MyParam)

if you want it to work predictably

回答3:

seeing many equality comparisons with [String]::Empty, you could use the [String]::IsNullOrWhiteSpace or [String]::IsNullOrEmpty static methods, like the following:

param(
    [string]$parameter = $null
)

# we know this is false
($null -eq $parameter)

[String]::IsNullOrWhiteSpace($parameter)
[String]::IsNullOrEmpty($parameter)
('' -eq $parameter)
("" -eq $parameter)

which yields:

PS C:\...> .\foo.ps1
False
True
True
True
True

回答4:

Simply do not declare the param's type if you want a $null value to remain:

Param(
    $stringParam
)

(None of the other solutions worked for me when declaring the type.)

回答5:

So it seems a default value of $null for parameters of type [string] defaults to empty string, for whatever reason.

Option 1

if ($stringParam) { ... }

Option 2

if ($stringParam -eq "") { ... }

Option 3

if ($stringParam -eq [String]::Empty) { ... }