Action-based object events sometimes lost

Question

Suppose an object referenced in some PS code is known to have fired 100 events between Register-ObjectEvent and Unregister-Event. Expectation would be for the host code to receive 100 events. However, it sometimes receives fewer than 100, often times a lot less.

I see this consistently happening in Windows 7 with PS v3, v4 and v5.1. The simple test case posted below appears to work under Windows 10, yet the failures in Windows 7 make me wonder if there is maybe something fundamentally wrong with the whole approach.

[ EDIT ]  The same happens in Windows 10 with PS v5.1, only less often than in Windows 7. It took a few thousand runs with $fired = 10000 for it to happen, but it eventually did. [ end EDIT ]

Minimal self-contained code to make it happen (shamelessly adapted from an answer here):

$src = @'
using System;

namespace Utils {
public static class StaticEventTest 
{
    public static event EventHandler Fired;

    public static void RaiseFired()
    {
        if (Fired != null) 
        { 
            Fired(typeof(StaticEventTest), EventArgs.Empty); 
        }
    }
}}
'@

Add-Type -TypeDefinition $src

$fired = 1000
$global:recvd = 0

$srcId = 'Fired'

$id = Register-ObjectEvent ([Utils.StaticEventTest]) Fired `
        -SourceIdentifier $srcId -Action { $global:recvd++ }

for ($i = 1; $i -le $fired; $i++) {
    [Utils.StaticEventTest]::RaiseFired()
}

Unregister-Event -SourceIdentifier $srcId
Receive-Job -Job $id -Wait -AutoRemoveJob

if ($fired -eq $global:recvd) {
    ("total {0} events" -f $fired)
} else {
    ("total {0} events fired - {1} received" -f $fired, $global:recvd)
}

Having the above saved as test.ps1 and run in a loop gives something like this (on Windows 7):

C:\etc>for /l %n in (1, 1, 10) do @powershell.exe  -executionPolicy remoteSigned -file test.ps1
total 1000 events
total 1000 events
total 1000 events
total 1000 events fired - 391 received
total 1000 events
total 1000 events
total 1000 events fired - 59 received
total 1000 events
total 1000 events fired - 199 received
total 1000 events

Any insghts much appreciated.

Answer 1

You are "losing" events because you end your script too early. Your event action defined by Register-ObjectEvent ([Utils.StaticEventTest]) Fired -SourceIdentifier $srcId -Action { $global:recvd++ } will be executed asynchronously to your script. If I repeat what you did, I get even worse results (maybe due to a virtual environment):

C:\Users\user\Desktop>for /l %n in (1, 1, 10) do @powershell.exe  -executionPolicy remoteSigned -file test.ps1
total 1000 events fired - 17 received
total 1000 events fired - 100 received
total 1000 events fired - 202 received
total 1000 events fired - 16 received
total 1000 events fired - 3 received
total 1000 events fired - 120 received
total 1000 events fired - 1 received
total 1000 events fired - 162 received
total 1000 events fired - 1 received
total 1000 events fired - 27 received

But when I add a Start-Sleep -Seconds 5 after your for-loop, I get this result (at least in my environment):

C:\Users\user\Desktop>for /l %n in (1, 1, 10) do @powershell.exe  -executionPolicy remoteSigned -file test.ps1
total 1000 events
total 1000 events
total 1000 events
total 1000 events
total 1000 events
total 1000 events
total 1000 events
total 1000 events
total 1000 events
total 1000 events

So all events you fired are there, no event is lost. But if you want to see the action taken on them right after you fired them, you have to wait until all (asynchronous) actions have been performed.

Of course, Start-Sleep with a fixed time span is not an option here, because you cannot know how long it takes for a system to execute all actions. But it shows the reason, why you think you are losing events.

A possible way to wait for your events would be this line after your for-loop:

while ($global:recvd -lt $fired) {Start-Sleep -Milliseconds 10}

This even works for 100.000 events per iteration (really tried):

C:\Users\user\Desktop>for /l %n in (1, 1, 10) do @powershell.exe  -executionPolicy remoteSigned -file test.ps1
total 100000 events
total 100000 events
total 100000 events
total 100000 events
total 100000 events
total 100000 events
total 100000 events
total 100000 events
total 100000 events
total 100000 events

Not a single event lost. This makes me believe that the firing and listening for events works as expected. I am not able to reproduce any counterproof.