Why $Null validation failed in powershell function?

Question

team!

I have validation script for parameter $Data. It fail when it get $null.

whats wrong?

[CmdletBinding()]
    Param (
        [Parameter(  HelpMessage = "PsObject data." )]
        [AllowNull()]
        [AllowEmptyCollection()]
        [AllowEmptyString()]
        [ValidateScript({
            if ( ( $_ -eq $null ) -or ( $_ -eq '' ) ){
                $true
            }
            else {
                (( !( $_.GetType().IsValueType ) ) -and ( $_ -isnot [string] ))
            }
        })]        
        $Data,
...

$UserObject = Show-ColoredTable -Data $null -View 'SamAccountName', 'Name', 'DistinguishedName', 'Enabled'  -Title "AD User list" -SelectMessage "Select AD user(s) to disable VPN access: " -AddRowNumbers -AddNewLine -SelectField "SamAccountName"

Your validation logic doesn't make too much sense - you want to [AllowEmptyString] but fail on input validation whenever any [string] is passed? — Mathias R. Jessen
– Mathias R. Jessen, Commented Nov 3, 2021 at 16:33
Also: If you parameter ($Data) is untyped (implies [object]), you don't need [AllowNull()], [AllowEmptyCollection()], and [AllowEmptyString()] - it'll work the same without them. — mklement0
– mklement0, Commented Nov 3, 2021 at 16:43
Corrected it. The same validation error. $data can be [string] or [psobject] or something else. It else, doesnt work without [AllowNull()], [AllowEmptyCollection()], [AllowEmptyString()]. — Alexey I. Kuzhel
– Alexey I. Kuzhel, Commented Nov 3, 2021 at 16:47
Yes - my comment was just an aside. The real problem is explained in Mathias' answer. — mklement0
– mklement0, Commented Nov 3, 2021 at 16:51

mklement0 · Accepted Answer · 2021-11-03 16:40:13Z

1

Most validation attributes are incompatible with [AllowNull()] because the first thing they all check - before your custom validation is invoked - is whether the input object is $null or not.

Move the validation logic inside the function body:

[CmdletBinding()]
Param (
    [Parameter(  HelpMessage = "PsObject data." )]
    [AllowNull()]
    [AllowEmptyCollection()]
    [AllowEmptyString()]
    $Data
)

# validate $Data here before the rest of the script/command

edited Nov 3, 2021 at 16:40

mklement0

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answered Nov 3, 2021 at 16:34

Mathias R. Jessen

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4 Comments

Alexey I. Kuzhel Over a year ago

It`s a good idea. My aim, to validate this, inside validatescript block.

mklement0 Over a year ago

@Alex, it's not just a good idea, it's your only option as of PowerShell 7.2, because, as the answer implies, the use of [ValidateScript()] alone - irrespective of the content of the script - unexpectedly prevents passing $null. This is arguably a bug, which was reported in May 2020 in GitHub issue #12557. A succinct repro: & { param([ValidateScript({ $true })] $Foo) } -Foo $null

Alexey I. Kuzhel Over a year ago

@Mathias R. Jessen, I think, and for Powershell 7.1 and 5.1 also.

Mathias R. Jessen Over a year ago

Indeed, this behavior stretches back to at least 5.1

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