Passing Arguments to Register-EngineEvent Action Scriptblock - powershell

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.

Answer 1

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