PowerShell Remoting $Using variable scope

Question

I have folder c:\test where I have three files: “file1”, “file2”, “file3”

Following script:

$remoteSession = New-PSSession -ComputerName localhost
$folder = "c:\test"
$exclude =@("c:\test\file1","c:\test\file2")

Invoke-Command -Session $remoteSession -ScriptBlock {    
    #$Using:exclude
    Get-ChildItem -Path $Using:folder -recurse | Where {$Using:exclude -notcontains $_.FullName}
}

Remove-PSSession $remoteSession 

Gives the result:

However, if I uncomment “$Using:exclude” I get the result:

Suddenly exclude list starts working properly

Answer 1

Just specifying $Using:exclude in the Where-Object cmdlet doesn't work because it is in a nested scriptblock.

In your case , the Using:folder works because it is a local variable directly passed to the Invoke-Command scriptblock.

But "Using:exclude is passed to a scriptblock for Where-Object, which is itself nested inside the scriptblock for Invoke-Command.

$Using allows to pass local variables to scriptblocks only one level deep, not to scriptblocks nested any further. This behaviour is not specific to the Where-Object scriptblock, any cmdlet which has a parameter taking a scriptblock behaves like this when it is inside a Invoke-Command scriptblock.

Unfortunately, I don't think this behaviour is documented.

By uncommenting $Using:exclude at the beginning of the Invoke-Command scriptblock, you are effectively declaring the variable $exclude inside the remote session. So, in this case, $exclude becomes a local variable inside the Invoke-Command scriptblock and can be passed one level further, to the nested Where-Object scriptblock.

That's why it works when you uncomment $Using:exclude at the beginning of the Invoke-Command scriptblock, it's a workaround for the behaviour of $Using.

For the official help information about this run :

Get-Help about_remote_variables -Full