# this function should be called when a new file is created
function foobar(){
  $form.BackColor = "black"
}

# set up runspace for async FileSystemWatcher
$Runspace = [runspacefactory]::CreateRunspace()
$PowerShell = [System.Management.Automation.PowerShell]::Create()
$PowerShell.runspace = $Runspace
$Runspace.Open()

[void]$PowerShell.AddScript({
  $logFile = 'C:powershelltest.log'
  $dirName = 'C:powershell'

  $hotFolder = New-Object IO.FileSystemWatcher $dirName -Property @{IncludeSubdirectories = $false;NotifyFilter = [IO.NotifyFilters]'FileName, LastWrite'}
  Register-ObjectEvent $hotFolder Created -SourceIdentifier FileCreated -Action {
  $name = $Event.SourceEventArgs.Name
  $path = $Event.SourceEventArgs.FullPath
  $changeType = $Event.SourceEventArgs.ChangeType
  $timeStamp = $Event.TimeGenerated

  Out-File -FilePath $logFile -Append -InputObject "The file '$name' was $changeType at $timeStamp"

  # this call does not work
  foobar
  }
})

$AsyncObject = $PowerShell.BeginInvoke()

# set up form
$form = New-Object System.Windows.Forms.Form
$form.ShowDisplay()