try catch is invoke-command

Question

I have a script block which goes to the Dc and try linking a selected gpo and ou:

forEach ($line in $listBox1.selecteditems) {
    try {
        Invoke-Command -Session $s -ScriptBlock {New-GPLink -Name $using:line -Target $using:SWITCH -ErrorAction Stop | Out-Null}
        $ResultsTextBox.AppendText("`r $line HAVE BEEN LINKED Successfully.`r") 
    } catch {
        $ResultsTextBox.AppendText("`n $line IS NOT LINKED TO THIS OU`n")
    }
}

Only problem here is the try catch wont work, what im trying to do is give the user information if he's trying to link an already linked gpo - ou. but what happens when i run this block is the user get output "... have been linked successfully" even the the link was already existed. and i will see an error output in the command line as follows:

The GPO named 'MSAVS-Policy1' is already linked to a Scope of Management with Path 'OU=Domain
Controllers,DC=shahar,DC=local'.
    + CategoryInfo          : InvalidArgument: (Microsoft.Group...ewGPLinkCommand:NewGPLinkCommand) [New-GPLink], Argu
   mentException
    + FullyQualifiedErrorId : UnableToCreateNewLink,Microsoft.GroupPolicy.Commands.NewGPLinkCommand
    + PSComputerName        : dc01

Whats am i doing wrong here?

Answer 1

Since you're executing the New-GPLink cmdlet remotely, we have 2 levels of error handling to consider - errors thrown by the commands inside the remote scriptblock, and then the errors (re-)thrown by Invoke-Command.

The try block is "blind" to what going on remotely, and can only catch the errors on the client, so Invoke-Command will also have to treat throw errors as terminating, so add the parameter -ErrorAction parameter to that as well:

try{
  Invoke-Command {...} -ErrorAction Stop
}
catch{
  # ...
}