How to pass $_ ($PSItem) in a ScriptBlock

Question

I'm basically building my own parallel foreach pipeline function, using runspaces.

My problem is: I call my function like this:

somePipeline | MyNewForeachFunction { scriptBlockHere } | pipelineGoesOn...

How can I pass the $_ parameter correctly into the ScriptBlock? It works when the ScriptBlock contains as first line

param($_)

But as you might have noticed, the powershell built-in ForEach-Object and Where-Object do not need such a parameter declaration in every ScriptBlock that is passed to them.

Thanks for your answers in advance fjf2002

EDIT:

The goal is: I want comfort for the users of function MyNewForeachFunction - they shoudln't need to write a line param($_) in their script blocks.

Inside MyNewForeachFunction, The ScriptBlock is currently called via

$PSInstance = [powershell]::Create().AddScript($ScriptBlock).AddParameter('_', $_)
$PSInstance.BeginInvoke()

EDIT2:

The point is, how does for example the implementation of the built-in function ForEach-Object achieve that $_ need't be declared as a parameter in its ScriptBlock parameter, and can I use that functionality, too?

(If the answer is, ForEach-Object is a built-in function and uses some magic I can't use, then this would disqualify the language PowerShell as a whole in my opinion)

EDIT3:

Thanks to mklement0, I could finally build my general foreach loop. Here's the code:

function ForEachParallel {
    [CmdletBinding()]
    Param(
        [Parameter(Mandatory)] [ScriptBlock] $ScriptBlock,
        [Parameter(Mandatory=$false)] [int] $PoolSize = 20,
        [Parameter(ValueFromPipeline)] $PipelineObject
    )

    Begin {
        $RunspacePool = [runspacefactory]::CreateRunspacePool(1, $poolSize)
        $RunspacePool.Open()
        $Runspaces = @()
    }

    Process {
        $PSInstance = [powershell]::Create().
            AddCommand('Set-Variable').AddParameter('Name', '_').AddParameter('Value', $PipelineObject).
            AddCommand('Set-Variable').AddParameter('Name', 'ErrorActionPreference').AddParameter('Value', 'Stop').
            AddScript($ScriptBlock)

        $PSInstance.RunspacePool = $RunspacePool

        $Runspaces += New-Object PSObject -Property @{
            Instance = $PSInstance
            IAResult = $PSInstance.BeginInvoke()
            Argument = $PipelineObject
        }
    }

    End {
        while($True) {
            $completedRunspaces = @($Runspaces | where {$_.IAResult.IsCompleted})

            $completedRunspaces | foreach {
                Write-Output $_.Instance.EndInvoke($_.IAResult)
                $_.Instance.Dispose()
            }

            if($completedRunspaces.Count -eq $Runspaces.Count) {
                break
            }

            $Runspaces = @($Runspaces | where { $completedRunspaces -notcontains $_ })
            Start-Sleep -Milliseconds 250
        }

        $RunspacePool.Close()
        $RunspacePool.Dispose()
    }
}

Code partly from MathiasR.Jessen, Why PowerShell workflow is significantly slower than non-workflow script for XML file analysis

Answer 1

The key is to define $_ as a variable that your script block can see, via a call to Set-Variable.

Here's a simple example:

function MyNewForeachFunction {
  [CmdletBinding()]
  param(
    [Parameter(Mandatory)]
    [scriptblock] $ScriptBlock
    ,
    [Parameter(ValueFromPipeline)]
    $InputObject
  )

  process {
    $PSInstance = [powershell]::Create()

    # Add a call to define $_ based on the current pipeline input object
    $null = $PSInstance.
      AddCommand('Set-Variable').
        AddParameter('Name', '_').
        AddParameter('Value', $InputObject).
      AddScript($ScriptBlock)

    $PSInstance.Invoke()
  }

}

# Invoke with sample values.
1, (Get-Date) | MyNewForeachFunction { "[$_]" }

The above yields something like:

[1]
[10/26/2018 00:17:37]

Answer 2

Maybe this can help. I'd normally run auto-generated jobs in parallel this way:

Get-Job | Remove-Job

foreach ($param in @(3,4,5)) {

 Start-Job  -ScriptBlock {param($lag); sleep $lag; Write-Output "slept for $lag seconds" } -ArgumentList @($param)

}

Get-Job | Wait-Job | Receive-Job

If I understand you correctly, you are trying to get rid of param() inside the scriptblock. You may try to wrap that SB with another one. Below is the workaround for my sample:

Get-Job | Remove-Job

#scriptblock with no parameter
$job = { sleep $lag; Write-Output "slept for $lag seconds" }

foreach ($param in @(3,4,5)) {

 Start-Job  -ScriptBlock {param($param, $job)
  $lag = $param
  $script = [string]$job
  Invoke-Command -ScriptBlock ([Scriptblock]::Create($script))
 } -ArgumentList @($param, $job)

}

Get-Job | Wait-Job | Receive-Job