Launch PowerShell scriptblock asynchronously without dependency on parent process - multithreading

Question

Below is a small PowerShell script that runs some PowerShell code asynchronously to show a dialog box (for the purpose of demonstrating my issue).
If I close the parent PowerShell process, the child process will also close and the dialog box disappears. Is there any way to launch a PowerShell scriptblock, complete with functions and arguments, asynchronously and without a dependence on the parent PowerShell process?
$testjob = [PowerShell]::Create().AddScript({ $a = new-object -comobject wscript.shell
$b = $a.popup('This is a test',5,'Test Message Box',1) })
$result = $testJob.BeginInvoke()
Update #2
I am trying to execute a script block, rather than an external script. The script block should use functions and variables from the parent script. The problem is, I can't pass those functions or variables in to the new process unless they are contained directly within the script block. Any idea if this is doable?
Function Show-Prompt {
Param ($title,$message)
$a = new-object -comobject wscript.shell
$b = $a.popup($message,5,$title,1)
}
$scriptContent = {Show-Prompt -Message 'This is a test' -Title 'Test Message Box'}
$scriptBlock = [scriptblock]::Create($scriptContent)
Start-process powershell -argumentlist "-noexit -command &{$scriptBlock}"

Answer 1

You can use an intermediate PowerShell process. There has to be a direct parent to process the return value from the async script. For instance, put your script in a file called popup.ps1 and try execute like so:
Start-Process PowerShell -Arg c:\popup.ps1
You might want to bump the timeout up a bit from say 5 to 10 seconds. You can close the original PowerShell and the popup will stay. When you close the popup (or it times out) the secondary PowerShell window will disappear.

Answer 2

You can do this with WMI. If you use Win32_Process to create the process, it will persist after you close PowerShell.
For instance:
invoke-wmimethod -path win32_process -name create -argumentlist calc

Answer 3

function GeneratePortableFunction {
param ([string] $name, [scriptblock] $sb)
$block = [ScriptBlock]::Create("return `${function:$name};");
$script = $block.Invoke();
$block = [ScriptBlock]::Create($script);
return ("function {0} {{ {1} }}" -f $name, $block.ToString());
}
function RemoteScript {
param ([string] $header, [string[]] $functions, [string] $footer)
$sb = New-Object System.Text.StringBuilder;
[void]$sb.Append("$header`n");
$functions | % {
[void]$sb.Append($_);
[void]$sb.Append("`n");
}
[void]$sb.Append($footer);
return [ScriptBlock]::Create($sb.ToString());
}
$fnc = #();
$fnc += GeneratePortableFunction -name "NameOfYourFunction1";
$fnc += GeneratePortableFunction -name "NameOfYourFunction2(CallsNameOfYourFunction1)";
$script = RemoteScript -header "param([int] `$param1)" -functions #($fnc) -footer "NameOfYourFunction2 -YourParameter `$param1;";
$p1 = 0;
$job = Start-Job -ScriptBlock $script -ArgumentList #($p1);
while($job.State -eq "Running")
{
write-host "Running...";
Start-Sleep 5;
}
$result = $job | Receive-Job;
Remove-Job -Id $job.Id;