Inconsistent behavior in powershell with null parameters

Question

I need to write a function in powershell that tells apart a 'parameter not being passed' from one passed with string empty (or any other string)

I wrote it like this:

function Set-X {
    param(
    [AllowNull()][string]$MyParam = [System.Management.Automation.Language.NullString]::Value
    )

    if ($null -ne $MyParam) { write-host 'oops' }
    else { write-host 'ok' }
}

If I call Set-X without parameters from ISE, it works as I expect and prints 'ok'.

But if I do that from the normal console, it prints 'oops'.

What is going on? What is the proper way to do it?

Have you tried looking at [string]::IsNullOrEmpty()? That will allow you to check for a null or empty string. There is also [string]::IsNullOrWhiteSpace() — Jason Snell
– Jason Snell, Commented Sep 11, 2017 at 17:21
What do you want to dispay if i type in : Set-X -MyParam "hey" ; Set-X -MyParam ; Set-X — ArcSet
– ArcSet, Commented Sep 11, 2017 at 17:39
I believe the problem is that you are casting your parameter to a [string]. As [string]$null -eq '' returns True vs $null -eq '' returns False — BenH
– BenH, Commented Sep 11, 2017 at 17:53
@ArcSet, I expect: Set-X -MyParam "hey" -> should display 'oops'; Set-X -> should display 'ok'; Set-X -MyParam -> I didn't think about this case, but since $MyParam is not [switch], I think it should result in some type of error in the function call — Alex
– Alex, Commented Sep 11, 2017 at 18:13

Mathias R. Jessen · Accepted Answer · 2017-09-11 20:33:53Z

2

Allowing the user to pass in a parameter argument value of $null does not change the fact that powershell will attempt to convert it to a [string].

Converting a $null value in powershell to a string results in an empty string:

$str = [string]$null
$null -eq $str # False
'' -eq $str    # True

(same goes for $null -as [string] and "$null")

Remove the type constraint on the MyParam parameter if you not only want to allow $null but also accept $null as a parameter value:

function Set-X {
    param(
    [AllowNull()]$MyParam = [System.Management.Automation.Language.NullString]::Value
    )

    if ($null -ne $MyParam) { write-host 'oops' }
    else { write-host 'ok' }
}

answered Sep 11, 2017 at 20:33

Mathias R. Jessen

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1 Comment

Alex Over a year ago

This is the correct answer, as also pointed out by BenH in his comment. But you also have to change the default assignment to simply $null, instead of [System.Management.Automation.Language.NullString]::Value

Alex · Accepted Answer · 2019-11-19 22:51:07Z

0

As Mathias and BenH have written, the culprit is casting $null to the [string] type, which results in an empty string:

 [string]$null -eq '' #This is True

But for the sample code in Mathias answer to work correctly we also have to replace

[System.Management.Automation.Language.NullString]::Value

with $null

function Set-X {
    param(
    [AllowNull()]$MyParam = $null
    )

    if ($null -ne $MyParam) { write-host 'oops' }
    else { write-host 'ok' }
}

answered Nov 19, 2019 at 22:51

Alex

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