Iterator variable `$_` is `$null` in process block when parameter is provided as argument instead of pipeline about powershell HOT 5 CLOSED

• $_ only has a meaningful value with pipeline input, not also when input is provided via arguments.

• Fundamentally, do not rely on $_ in the process block of advanced script functions / scripts that explicitly declare pipeline-binding parameters - only use the latter.

In your case, this means that your process block should (a) only act on $Assertions, and (b) be aware of the fact that $Assertions, due to its declaration as [hashtable[]], is always an array of [hashtable] instances, even if [hashtable] instances are provided one by one via the pipeline.

from powershell.

Thanks a lot for enlightening me, @mklement0!

Would you agree that this important fact should be mentioned in the docs at
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_functions_advanced_methods#process

about Functions Advanced Methods - PowerShell

Describes how functions that specify the `CmdletBinding` attribute can use the methods and properties that are available to compiled cmdlets.

PS: Yes I'm intentionally using [hashtable[]], so I don't need to make this harmonization (if ($Assertions -isnot [array]) $Assertions = @($Assertions)) in my code.

from powershell.

Glad to hear it helped, @SetTrend , and I agree re the docs; see:

• MicrosoftDocs/PowerShell-Docs#10944

from powershell.

The documentation for the process block says:

The automatic variable $_ or $PSItem contains the current object in the pipeline for use in the process block.

Since you are calling the function with a parameter, there is nothing in the pipeline. The function needs to handle the parameter variable. This version of the function works for both pipeline input and parameter input.

function Test-Something {
    param (
        [Parameter(Mandatory = $true, Position = 0, ValueFromPipeline = $true)]
        [hashtable[]]$Assertions
    )

    begin { [hashtable[]]$retValue = @{} }

    process {
        foreach ($assertion in $Assertions) {
            $retValue += $assertion
        }
    }

    end { $retValue }
}

from powershell.

For context, @sdwheeler:

• This previous comment already explains the issue, and it makes it clear that you shouldn't use $_ in the process block of advanced functions - use the parameter (variable) declared to be pipeline-binding.

  • MicrosoftDocs/PowerShell-Docs#10944 (comment) explains why in more detail.

• @SetTrend had run into the original problem because had relied on the documentation, which confusingly actively suggests using $_ (which is only appropriate for non-advanced functions).

  • The fact that the passage you quote from the documentation only mentions the pipeline is technically correct, but doesn't prevent conceptual confusion, as evidenced by the case at hand.

• He then suggested that the documentation be amended, causing me to create MicrosoftDocs/PowerShell-Docs#10944, which you've recently closed as resolution-wont-fix, which is inappropriate in my estimation.

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