How to pass variable by reference to Powershell job or runspace? - powershell

Question

I have Powershell job.
$cmd = {
param($a, $b)
$a++
$b++
}
$a = 1
$b = 2
Start-Job -ScriptBlock $cmd -ArgumentList $a, $b
How to pass $a and $b by a reference so when the job is done they will be updated? Alternatively how to pass variables by reference to runspaces?

Answer 1

Simple sample I just wrote (don't mind the messy code)
# Test scriptblock
$Scriptblock = {
param([ref]$a,[ref]$b)
$a.Value = $a.Value + 1
$b.Value = $b.Value + 1
}
$testValue1 = 20 # set initial value
$testValue2 = 30 # set initial value
# Create the runspace
$Runspace = [runspacefactory]::CreateRunspace()
$Runspace.ApartmentState = [System.Threading.ApartmentState]::STA
$Runspace.Open()
# create the PS session and assign the runspace
$PS = [powershell]::Create()
$PS.Runspace = $Runspace
# add the scriptblock and add the argument as reference variables
$PS.AddScript($Scriptblock)
$PS.AddArgument([ref]$testValue1)
$PS.AddArgument([ref]$testValue2)
# Invoke the scriptblock
$PS.BeginInvoke()
After running this the for the testvalues are updated since they are passed by ref.

Answer 2

Passing parameters by reference is always awkward in PowerShell, and probably won't work for PowerShell jobs anyway, as #bluuf pointed out.
I would probably do something like this instead:
$cmd = {
Param($x, $y)
$x+1
$y+1
}
$a = 1
$b = 2
$a, $b = Start-Job -ScriptBlock $cmd -ArgumentList $a, $b |
Wait-Job |
Receive-Job
The above code passes the variables $a and $b to the scriptblock and assigns the modified values back to the variables after receiving the job output.

Answer 3

a more comprehensive script with example .
it should also include ability to pass $host or something, to make write-host from the passed script, output to the console . but i don't have time to figure out how to do this .
$v = 1
function newThread ([scriptblock]$script, [Parameter(ValueFromPipeline)]$param, [Parameter(ValueFromRemainingArguments)]$args) {
process {
$Powershell = [powershell]::Create()
$Runspace = [runspacefactory]::CreateRunspace()
# allows to limit commands available there
# $InitialSessionState = InitialSessionState::Create()
# $Runspace.InitialSessionState = $InitialSessionState
$Powershell.Runspace = $Runspace
$null = $Powershell.AddScript($script)
$null = $Powershell.AddArgument($param)
foreach ($v_f in $args) {
$null = $Powershell.AddArgument($v_f)
}
$Runspace.Open()
$Job = $Powershell.BeginInvoke()
[PSCustomObject]#{
Job=$Job
Powershell=$Powershell
}
}
}
$script = {
param([ref]$v,$v2)
$v.Value++
$v2
}
$thread = newThread $script ([ref]$v) 3
do {} until ($thread.Job.IsCompleted)
$v1 = $thread.Powershell.EndInvoke($thread.Job)
$thread.Powershell.Dispose()
write-host "end $($v,$v1[0])"