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Describing Object Events

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Today we take a look at the final member of an object, Events. Events are something that I use mostly if I need to put a graphical interface on top of my PowerShell code. You can also use them in your non-graphical PowerShell solutions. An event is generated when something happens to an object. So Properties describe an object. Methods are actions the object can take.  Events are when something happens to the object.  Let’s take a look at the member information for a Process Object and focus on the events.

PS C:\> Get-Process | Get-Member -MemberType Event


   TypeName: System.Diagnostics.Process

Name               MemberType Definition                                                                               
----               ---------- ----------                                                                              
Disposed           Event      System.EventHandler Disposed(System.Object, System.EventArgs)                           
ErrorDataReceived  Event      System.Diagnostics.DataReceivedEventHandler ErrorDataReceived(System.Object, System.Di...
Exited             Event      System.EventHandler Exited(System.Object, System.EventArgs)                             
OutputDataReceived Event      System.Diagnostics.DataReceivedEventHandler OutputDataReceived(System.Object, System.D...


We can see that there is an event called Exited.  We are going to subscribe to the Exited event of a process object. Here is what the MSDN documentation on System.Diagnostic.Process says about its events.

Go ahead an open an instance of Notepad.

PS C:\> Start-Process -FilePath Notepad

Save the instance of the Notepad process in a variable for easy reference.

PS C:\> $Proc = Get-Process -Name notepad

Now subscribe to your objects event. (I used the backtick character at the end of the first 2 lines to be careful not to miss it.)

PS C:\> Register-ObjectEvent -InputObject $Proc `
                     -EventName exited `
                     -SourceIdentifier notepad `
                     -Action {Write-Host "Process ended" -BackgroundColor DarkRed}

Id     Name            PSJobTypeName   State         HasMoreData     Location             Command                 
--     ----            -------------   -----         -----------     --------             -------                 
60     notepad                         NotStarted    False                                Write-Host "Process en...

We placed an action to take when this process ends.  It will display “Process ended” with a dark red background.  We can see that this is a background job.

PS C:\> Get-Job

Id     Name            PSJobTypeName   State         HasMoreData     Location             Command                 
--     ----            -------------   -----         -----------     --------             -------                 
60     notepad                         NotStarted    False                                Write-Host "Process en...

We can also see the event that we are currently subscribed to.

PS C:\> Get-EventSubscriber -SourceIdentifier Notepad


SubscriptionId   : 8
SourceObject     : System.Diagnostics.Process (notepad)
EventName        : exited
SourceIdentifier : notepad
Action           : System.Management.Automation.PSEventJob
HandlerDelegate  :
SupportEvent     : False
ForwardEvent     : False

Go ahead and stop the Notepad process.

PS C:\> Stop-Process -Name notepad

Process ended

You can see that our action executed.
Even though the process is closed and no longer exists, you are still subscribed to it.

PS C:\> Get-EventSubscriber


SubscriptionId   : 8
SourceObject     : System.Diagnostics.Process (notepad)
EventName        : exited
SourceIdentifier : notepad
Action           : System.Management.Automation.PSEventJob
HandlerDelegate  :
SupportEvent     : False
ForwardEvent     : False

You can unsubscribe to it.

PS C:\> Get-EventSubscriber -SourceIdentifier Notepad | Unregister-Event


That is how we subscribe to events in PowerShell.  Tomorrow, I’m going to show you how to do use events with SAPIEN PowerShell Studio.  You can get a trial version from here (https://www.sapien.com/software/powershell_studio) .  Just click on the Try It link on the right.
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