Passing Arguments to Register-EngineEvent Action Scriptblock

Question

I want to catch a PowerShell script as it finishes so that I can do some final processing before it stops. Register-EngineEvent -SourceIdentifier PowerShell.Exiting seems to be the way to do it but it does not work for anything other than trivial applications.

Simple examples work. For example:

Register-EngineEvent -SourceIdentifier PowerShell.Exiting -Action {write-host "This is trivial"}

will print "This is trivial" when the script finishes. However, any action block that needs data passed to it does not get that data. For example:

Register-EngineEvent -SourceIdentifier PowerShell.Exiting -Action {write-host ">> " $($event.MessageData)} -MessageData "This doesn't work"

will only print the two leading angle brackets and not the string "This doesn't work".

Note that I'm calling the script from a Command Prompt (cmd.exe) and it is there that the Write-Host ultimately prints, after PowerShell has exited.

Furthermore, with the exception of the $Event automatic variables, all the others such as $Sender, $Args, $EventArgs etc. are not populated. Also some of the properties of $Event are not populated. For example, $Event.ComputerName prints nothing, but $Event.TimeGenerated prints the current date and time. My computer has a name.

I have included a tiny example program which demonstrates either that I am doing it wrong or that there is some limitation in what can be done with Register-EngineEvent maybe it is even a bug I suppose.

I have spent quite a lot of time searching web sites but I haven't found any examples where they are passing data to an action block for Register-EngineEvent.

sleep 1
write-host "Starting"
sleep 1
write-host "Finished"
Register-EngineEvent -SourceIdentifier PowerShell.Exiting -MessageData "This doesn't work" -Action {write-host ">> " $($event.MessageData)}
sleep 2

I would expect that little script to print "Starting" followed 1 second later by "Finished" and then, after a further 2 seconds the event handler to print ">> This doesn't work". I do get the first two messages but all I get from the handler is ">>".

I am running on Windows 10 with Powershell V5. I am not running it in ISE. I use the command line powershell -file .\try6.ps1 where try6.ps1 is the script I've included above.

If anyone can suggest what I am doing wrong or alternative ways of doing this that would be great but even if it is just that it is a known bug or that I have misunderstood what PowerShell.exiting or Register-EngineEvent are and they can't be used in the way I am trying that would be very helpful as well.

So you're trying to Write-host back into the same PS session you're exiting from? I can't see how that is supposed to work. What if you write out your message data to file? If that works better, there's your problem. — LeeM
– LeeM, Commented Mar 26, 2019 at 4:07
@Trix: Colin is calling the PS script from cmd.exe, which is where the Write-Host output prints after the PS process exits - I've updated the question to make that clearer. — mklement0
– mklement0, Commented Mar 26, 2019 at 21:32
Yet another argument for abolishing CMD wrappers for PS scripts. As time goes on, I really don't get why people still do this. — LeeM
– LeeM, Commented Mar 27, 2019 at 0:27

mklement0 · Accepted Answer · 2021-09-17 18:50:14Z

Note: While this answer contains hopefully useful background information, it does not solve the OP's problem of wanting to pass custom event data to a PowerShell.Exiting event handler.

What Colin encountered is apparently a known bug: see GitHub issue #7000.

Generally, just to clarify: the PowerShell.Exiting event only fires on exiting a PowerShell session, not a script run inside a session.

Using it in the running session itself (as opposed to using it in a remote session that forwards the event to the caller) limits you to:

taking behind-the-scenes action when the session ends
using Write-Host to write output that the calling process potentially sees (use of implicit output or Write-Output is no longer an option, because PowerShell's output streams are no longer available at the time the event fires).

You're running a script locally via PowerShell's CLI, powershell.exe which means that the limitations above apply to you.

The automatic event-related variables documented in about_Automatic_Variables do not seem to contain much information when the event fires, as the following command demonstrates:

PS> powershell -c '$null = 
     Register-EngineEvent -SourceIdentifier PowerShell.Exiting -Action {
      $Event, $EventSubscriber, $Sender, $EventArgs, $Args | Out-string | Write-Host 
     }'
ComputerName     :
RunspaceId       : 1a763430-5cd8-4b74-aaaf-7a90e514518d
EventIdentifier  : 1
Sender           :
SourceEventArgs  :
SourceArgs       : {}
SourceIdentifier : PowerShell.Exiting
TimeGenerated    : 3/26/19 12:15:48 AM
MessageData      :

SubscriptionId   : 1
SourceObject     :
EventName        :
SourceIdentifier : PowerShell.Exiting
Action           : System.Management.Automation.PSEventJob
HandlerDelegate  :
SupportEvent     : False
ForwardEvent     : False

$Event exists, and provides the runspace ID and a time stamp reflecting the time of the event.
- $Event.ComputerName is presumably only populated if the event was forwarded (via Register-EngineEvent's -Forward switch) from a different computer in the context of remoting; if the property is empty, the implication is that the event fired on the local machine.
$EventSubscriber exists, but doesn't contain any useable information.
$EventArgs, $Sender and $Args are not populated.

Given the above, you could streamline your output to only contain the time stamp of the event and the local computer name's name:

PS> powershell -c '
     $null = 
       Register-EngineEvent -SourceIdentifier PowerShell.Exiting -Action {
       [pscustomobject] @{ TimeGenerated = $Event.TimeGenerated; ComputerName = $env:COMPUTERNAME }| Format-List | Out-String | Write-Host 
     }
     # ... call your script 
     .\try6.ps1
   '


TimeGenerated : 3/26/2019 8:57:53 AM
ComputerName  : Workstation10

Thanks for your help and advice and especially for the link to the bug. I did wonder if it was by design since the programmer might argue that there is little point in populating the automatic variable as the session is ending but I see from the bug report that it isn't just for PowerShell.Exiting that this problem exists. Anyway, I have done what I actually wanted to do a different way and that is working fine so all is well. Thanks also to @Trix for the suggestions. Strangely it does work. I did wonder if that was the problem too but it seems not.

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