How to call a powershell function within the script from Start-Job?

Question

I saw this question and this question but couldn't find solution to my problem.

So here is the situation: I have two functions DoWork and DisplayMessage in a script (.ps1) file. Here is the code:

### START OF SCRIPT ###
function DoWork
{
  $event = Register-EngineEvent -SourceIdentifier NewMessage -Action {
      DisplayMessage($event.MessageData)
  }

  $scriptBlock =  {

    Register-EngineEvent -SourceIdentifier NewMessage -Forward

    $message = "Starting work"
    $null = New-Event -SourceIdentifier NewMessage -MessageData $message

    ### DO SOME WORK HERE ###

    $message = "Ending work"
    $null = New-Event -SourceIdentifier NewMessage -MessageData $message

    Unregister-Event -SourceIdentifier NewMessage
  }

  DisplayMessage("Processing Starts")

  $array = @(1,2,3)
  foreach ($a in $array)
  {
      Start-Job -Name "DoActualWork" $ScriptBlock -ArgumentList $array | Out-Null
  }

  #$jobs = Get-Job -Name "DoActualWork"
  While (Get-Job -Name "DoActualWork" | where { $_.State -eq "Running" } )
  {
      Start-Sleep 1
  }

  DisplayMessage("Processing Ends")

  Get-Job -Name "DoActualWork" | Receive-Job
}

function DisplayMessage([string]$message)
{
    Write-Host $message -ForegroundColor Red
}

DoWork

### END OF SCRIPT ###

I am creating 3 background jobs (using $array with 3 elements) and using event to pass messages from background jobs to the host. I would expect the powershell host to display "Processing Starts" and "Processing Ends" 1 time and "Starting work" and "Ending work" 3 times each. But instead I am not getting "Starting work"/"Ending work" displayed in console.

Event is also treated as a job in powershell, so when I do Get-Job, I can see following error associated with the event job:

{The term 'DisplayMessage' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.}

My question: How can we reuse (or reference) a function (DisplayMessage in my case) defined in the same script from where I am calling Start-Job? Is it possible? I know we can use -InitializationScript to pass functions/modules to Start-Job, but I do not want to write DisplayMessage function twice, one in script and another in InitializationScript.

Tudela

Answer 1

Background jobs are run in a seperate process, so the jobs you create with Start-Job can not interact with functions unless you include them in the $scriptblock.

Even if you included the function in the $scripblock, Write-Host would not output it's content to the console until you used Get-Job | Receive-Job to recieve the jobs result.

EDIT The problem is that your DisplayMessage function is in a local script-scope while your eventhandler runs in a different parent scope(like global which is the session scope), so it can't find your function. If you create the function in the global scope and call it from the global scope, it will work.

I've modified your script to do this now. I've also modified to scriptblock and unregistered the events when the script is done so you won't get 10x messages when you run the script multiple times :-)

Untitled1.ps1

function DoWork
{
  $event = Register-EngineEvent -SourceIdentifier NewMessage -Action {
      global:DisplayMessage $event.MessageData
  }

  $scriptBlock =  {

    Register-EngineEvent -SourceIdentifier NewMessage -Forward

    $message = "Starting work $args"
    $null = New-Event -SourceIdentifier NewMessage -MessageData $message

    ### DO SOME WORK HERE ###

    $message = "Ending work $args"
    $null = New-Event -SourceIdentifier NewMessage -MessageData $message

    Unregister-Event -SourceIdentifier NewMessage
  }

  DisplayMessage("Processing Starts")

  $array = @(1,2,3)
  foreach ($a in $array)
  {
      Start-Job -Name "DoActualWork" $ScriptBlock -ArgumentList $a | Out-Null
  }

  #$jobs = Get-Job -Name "DoActualWork"
  While (Get-Job -Name "DoActualWork" | where { $_.State -eq "Running" } )
  {
      Start-Sleep 1
  }

  DisplayMessage("Processing Ends")

  #Get-Job -Name "DoActualWork" | Receive-Job
}

function global:DisplayMessage([string]$message)
{
    Write-Host $message -ForegroundColor Red
}

DoWork

Get-EventSubscriber | Unregister-Event

Test

PS > .\Untitled1.ps1
Processing Starts
Starting work 1
Starting work 2
Ending work 1
Ending work 2
Starting work 3
Ending work 3
Processing Ends

Offertory

Thanks. I know Start-Job can't interact with outer functions, hence this questions. Also, I know Write-Host can't output to console directly, thats why I am using event to do this. In my case, I am able to write to host but only if I use "Write-Host" directly in the event but not when calling it from a function. – Tudela

Okey, I guess I missread your question, sorry about that. : ) See the new content in the end of my question. Is that your current problem? – Offertory

Indeed it was a scope issue. Thanks, your solution solved my problem. – Tudela

Answer 2

An easy way to include local functions in a background job:

$init=[scriptblock]::create(@"
function DoWork {$function:DoWork}
"@)

Start-Job -Name "DoActualWork" $ScriptBlock -ArgumentList $array -InitializationScript $init | Out-Null

Thessalonian

I like your way of including the local functions in background job! But it didn't solve my problem (not sure why :( ). I was able to solve my problem using @Graimer solution above. – Tudela

I got this to work by using the {} brackets to create the script block instead of the "@ @" operator as they will not interrupt the variables correctly. Example: $init= { function DoWork {$function:DoWork} } Then call it the same way using the Start-job & -InitializationScript. – Henbane

I got mjolinor's solution to work, but in my case, just including DoWork via the init argument didn't work, I also needed to include any functions/variables DoWork referenced, etc, recursively. – Sundown

Answer 3

I got it to work as follows;

function CreateTable {
    $table = "" | select ServerName, ExitCode, ProcessID, StartMode, State, Status, Comment
    $table
}

$init=[scriptblock]::create(@"
  function CreateTable {$function:createtable}
"@)

Queue