Powershell - Passing multiple arraylists to Invoke-Command block

Question

I am trying to write a powershell script that will tell me if a computer in my network is on or off, and if it is on, if there is anyone logged in. Currently I have:

# Create some empty arraylists                                                               
$availablecomputers = New-Object System.Collections.ArrayList
$unavailablecomputers = New-Object System.Collections.ArrayList
$usersloggedon = New-Object System.Collections.ArrayList

#Check connectivity for each machine via Test-WSMan
foreach ($computer in $restartcomputerlist)
{
    try 
    {
    Test-WSMan -ComputerName $computer -ErrorAction Stop |out-null
    Invoke-Command `
    -ComputerName  $computer `
    -ScriptBlock `
    {
        if
        ((Get-WmiObject win32_computersystem).username -like "AD\*")
        {
            $args[0] += $computer
        }
        else 
        {
            $args[1] += $computer
        }
    } `
    -ArgumentList (,$usersloggedon), (,$availablecomputers)
    }
    catch 
    {
    $unavailablecomputers += $computer 
    }
}

So far, if the computer is not on, it works correctly. However, if it is on, $computer won't be added to $usersloggedon or $availablecomputers. Any help would be appreciated.

$args only exist in the context of the ScriptBlock, assigning to it won't modify or "write back to" the variables you pass as arguments — Mathias R. Jessen
– Mathias R. Jessen, Commented Feb 19, 2015 at 22:35

briantist · Accepted Answer · 2015-02-20 20:17:02Z

1

@Mathias is correct; variables you pass into the scriptblock are passed by value (serialized), not by reference, so you can't update them and change the original object.

To return values from the scriptblock, use Write-Object or just simply "use" the value (Write-Object $env:COMPUTERNAME is the same as just doing $env:COMPUTERNAME).

For your specific situation, consider returning an object that contains the information you want:

$computers = @()

#Check connectivity for each machine via Test-WSMan
foreach ($computer in $restartcomputerlist)
{
    try 
    {
    Test-WSMan -ComputerName $computer -ErrorAction Stop |out-null
    $computers += Invoke-Command -ComputerName $computer -ScriptBlock {
        $props = @{
            Name = $env:COMPUTERNAME
            Available = $true
            UsersLoggedOn = ((Get-WmiObject win32_computersystem).username -like "AD\*")
        }
        New-Object PSObject -Property $props
    }
    }
    catch 
    {
    $props = @{
        Name = $computer
        Available = $false
        UsersLoggedOn = $false
    }
    $computers += New-Object PSObject -Property $props 
    }
}
$computers # You can now use this with Select-Object, Sort-Object, Format-* etc.

edited Feb 20, 2015 at 20:17

answered Feb 19, 2015 at 23:48

briantist

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2 Comments

jjia Over a year ago

Thanks! Yeah, I figured it was passed by value, so I was trying to figure out if there was a way to make it pass by reference. Your code worked beautifully, except that the -Properties part had to be changed to -Property.

briantist Over a year ago

D'oh, that's what I get for not testing the code before posting it! (Fixed)

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