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Registering for WMI Events in PowerShell

An alternate title might be ‘Running PowerShell Code ONLY when the power state changes’, because that was the very interesting task I received from my customer this week.

It was honestly too cool of a StackOverflow answer NOT to share, so here it goes, you can vote for it here if you thought it was worth-while.

If you want your code to trigger only when the System Power State changes, as described here, use this code.

Register-WMIEvent -query "Select \* From Win32\_PowerManagementEvent" \` -sourceIdentifier "Power" \` -action { #YourCodeHere } 

Now, this will trigger whenever the power state changes, whether you plug the device in, OR unplug it. So you might further want to stop and pause to ask the question:

Am I on power or not?

Fortunately we can use the WMI Class Win32_BatteryStatus to detect if we’re charging or not, so here’s the full construct that I use to ONLY run an operation when a power event changes, and then only if I’m no longer on Power.

Locking the workstation when the system is unplugged

Register-WMIEvent -query "Select * From Win32_PowerManagementEvent" `
  -sourceIdentifier "Power" `
  -action {
      if ([BOOL](Get-WmiObject -Class BatteryStatus -Namespace root\wmi).PowerOnLine ){
         #Device is plugged in now, do this action
         write-host "Power on!"
     }
    else{
        #Device is NOT plugged in now, do this action
        write-host "Now on battery, locking..."
        [NativeMethods]::LockWorkStation()
     }

If you’re curious how this looks in real time

Using PowerShell to register for a WMI event, to lock a workstation on power state change pic.twitter.com/JtJWDosA4b

— Stephen Owen 🦊Deploy (@FoxDeploy) December 15, 2016

Registering for device events

It can also be useful to have your code wait for something to happen with devices, such as running an action when a device is added or removed. To do this, use this code.

#Register for power state change
#Where TargetInstance ISA 'Win32_Process'"
Register-WMIEvent -query "Select * From Win32_DeviceChangeEvent where EventType = '2'" `
-sourceIdentifier "Power" `
-action {#Do Something when a device is added
Write-host "Device added at $(Get-date)"
} 

You might also want to do an action if a device is removed instead, so use this table to choose which event is right for you. Read more about it here.

What else can I wait for?

Not only these, but you can trigger your code to execute on a variety of useful WMI Events, all of which can be seen in this image below!