Programmatically Detecting when a VM Changes State

Nice approach can be found here.

# WMI Query that specifies what events we will watch for

$Query = "Select * from __InstanceModificationEvent within 3 where TargetInstance ISA 'MSVM_ComputerSystem' `

          and TargetInstance.EnabledState <> PreviousInstance.EnabledState"

# Script block that will run whenever the event fires

[ScriptBlock]$Action = {

   $VMName = $Event.SourceEventArgs.NewEvent.TargetInstance.ElementName

   switch ($Event.SourceEventArgs.NewEvent.TargetInstance.EnabledState)

      {

        2 {$vmState = "running"}

        3 {$vmState = "turned off"}

        9 {$vmState = "paused"}

        6 {$vmState = "in a saved state"}

        10 {$vmState = "starting"}

        4 {$vmState = "stopping"}

        default {$vmState = "in an unknown state..."}

       }

   if ($Event.SourceEventArgs.NewEvent.TargetInstance.EnabledState -eq 1)

      {$vmState = $Event.SourceEventArgs.NewEvent.TargetInstance.OtherEnabledState}

   write-host "The virtual machine '$($vmName)' is now $($vmState)."}

# Register for the events

Register-WMIEvent -Query $Query -Action $Action -Namespace root\virtualization\v2

# To clean up - run "Get-EventSubscriber | Unregister-Event"