Powershell is reanimating variables in Scripts - powershell

Question

Good evening
I have this Problem with this Version
PS C:\temp> $PSVersionTable.PSVersion.Major
4
This is a really strange problem...
despite of initialized variables, PowerShell script is somehow able to reuse variable values from previous invocations.
The script is simple; to show the problem, I work with a list of virtual machines:
Read all Virtual Machines into an Array
Select the 1st Element in the Array
Add a new Property to the Object from step 2
The Problem: if I run the script a second time, the new Property is already there - despite all Variables are initialized. If I start the Script in a new session, the new Property is missing on the 1st run, afterwards it is already there.
Here is the simple Code:
Set-StrictMode -Version 2.0
# Read all Virtual Machines into an Array
$AllVMs = #()
$AllVMs = Get-VM
# Get the 1st Virtual Machine
$VM = $null
$VM = $AllVMs[0]
# Prepare my Property
$MyList = #()
$MyList += "Test"
# If the Property already exists, just add my List
if ($VM.PSobject.Properties.Name -match "MyList") {
$VM.MyList += $MyList
} else {
# My Property does not exist: create it
$VM | Add-Member –MemberType NoteProperty –Name MyList –Value ($MyList)
}
# Give Back my VM Object
$VM
To test the script, I just count the number of MyList-Elements:
PS C:\temp> $result = c:\temp\testvar.ps1
PS C:\temp> $result.MyList.Count
1
PS C:\temp> $result = c:\temp\testvar.ps1
PS C:\temp> $result.MyList.Count
2
…
Does somone can help me with this Problem?
Thanks a lot for any help!!
Kind regards,
Tom

Answer 1

I have asked this question to 'the scripting guy' Forum, too.
I've got two great answers:
From jrv:
You are not exiting from PowerShell. The VM object is dynamic to the session. It persists until you close PowerShell. Some objects are like this. The code base drags them I to PowerShell and they remain cached. I suspect this is what is happening here.
From Evgenij Smirnov:
Hi,
this appears to be specific to the VM object. If I substitute Get-VM by Get-Process or Get-ChildItem c:\ I do not experience this behaviour. If I select a new VM every time I run the script, it does not retain the property. On the other hand, if I do (Get-VM)[0].MyList after running the script four times, I get four entries.
So this persistence is obviously built into the Hyper-V module, the custom property getting added to the instance of the VM object itself. So you could initialize MyTest to empty on the whole VM collection like this:
$AllVMs | foreach {
if ($_.PSobject.Properties.Name -match "MyList") {
$_.MyList = #()
}
}
Kind regards, Tom