Verify a function in PowerShell has run succesfully

I'm writing a script to backup existing bit locker keys to the associated device in Azure AD, I've created a function which goes through the bit locker enabled volumes and backs up the key to Azure however would like to know how I can check that the function has completed successfully without any errors. Here is my code. I've added a try and catch into the function to catch any errors in the function itself however how can I check that the Function has completed succesfully - currently I have an IF statement checking that the last command has run "$? - is this correct or how can I verify please?

    function Invoke-BackupBDEKeys {

        ##Get all current Bit Locker volumes - this will ensure keys are backed up for devices which may have additional data drives
        $BitLockerVolumes = Get-BitLockerVolume | select-object MountPoint
        foreach ($BDEMountPoint in $BitLockerVolumes.mountpoint) {

            try {
            #Get key protectors for each of the BDE mount points on the device
            $BDEKeyProtector = Get-BitLockerVolume -MountPoint $BDEMountPoint | select-object -ExpandProperty keyprotector
            #Get the Recovery Password protector - this will be what is backed up to AAD and used to recover access to the drive if needed
            $KeyId = $BDEKeyProtector | Where-Object {$_.KeyProtectorType -eq 'RecoveryPassword'}
            #Backup the recovery password to the device in AAD
            BackupToAAD-BitLockerKeyProtector -MountPoint $BDEMountPoint -KeyProtectorId $KeyId.KeyProtectorId
            }
             catch {
                 Write-Host "An error has occured" $Error[0] 
            }
        }
    }     

#Run function
    Invoke-BackupBDEKeys

if ($? -eq $true) {

    $ErrorActionPreference = "Continue"
    #No errors ocurred running the last command - reg key can be set as keys have been backed up succesfully
    $RegKeyPath = 'custom path'
    $Name = 'custom name'
    New-ItemProperty -Path $RegKeyPath -Name $Name -Value 1 -Force
    Exit
}
 else {
    Write-Host "The backup of BDE keys were not succesful"
    #Exit
}

Solution

Unfortunately, as of PowerShell 7.2.1, the automatic $? variable has no meaningful value after calling a written-in-PowerShell function (as opposed to a binary cmdlet) . (More immediately, even inside the function, $? only reflects $false at the very start of the catch block, as Mathias notes).
- If PowerShell functions had feature parity with binary cmdlets, then emitting at least one (non-script-terminating) error, such as with Write-Error, would set $? in the caller's scope to $false, but that is currently not the case.
- You can work around this limitation by using $PSCmdlet.WriteError() from an advanced function or script, but that is quite cumbersome. The same applies to $PSCmdlet.ThrowTerminatingError(), which is the only way to create a statement-terminating error from PowerShell code. (By contrast, the throw statement generates a script-terminating error, i.e. terminates the entire script and its callers - unless a try / catch or trap statement catches the error somewhere up the call stack).
- See this answer for more information and links to relevant GitHub issues.
As a workaround, I suggest:
- Make your function an advanced one, so as to enable support for the common -ErrorVariable parameter - it allows you to collect all non-terminating errors emitted by the function in a self-chosen variable.
  - Note: The self-chosen variable name must be passed without the $; e.g., to collection in variable $errs, use -ErrorVariable errs; do NOT use Error / $Error, because $Error is the automatic variable that collects all errors that occur in the entire session.
  - You can combine this with the common -ErrorAction parameter to initially silence the errors (-ErrorAction SilentlyContinue), so you can emit them later on demand. Do NOT use -ErrorAction Stop, because it will render -ErrorVariable useless and instead abort your script as a whole.
- You can let the errors simply occur - no need for a try / catch statement: since there is no throw statement in your code, your loop will continue to run even if errors occur in a given iteration.
  - Note: While it is possible to trap terminating errors inside the loop with try / catch and then relay them as non-terminating ones with $_ | Write-Error in the catch block, you'll end up with each such error twice in the variable passed to -ErrorVariable. (If you didn't relay, the errors would still be collected, but not print.)
- After invocation, check if any errors were collected, to determine whether at least one key wasn't backed up successfully.
- As an aside: Of course, you could alternatively make your function output (return) a Boolean ($true or $false) to indicate whether errors occurred, but that wouldn't be an option for functions designed to output data.

Here's the outline of this approach:

function Invoke-BackupBDEKeys {
  # Make the function an *advanced* function, to enable
  # support for -ErrorVariable (and -ErrorAction)
  [CmdletBinding()]
  param()

  # ...
  foreach ($BDEMountPoint in $BitLockerVolumes.mountpoint) {

      # ... Statements that may cause errors.
      # If you need to short-circuit a loop iteration immediately
      # after an error occurred, check each statement's return value; e.g.:
      #      if (-not $BDEKeyProtector) { continue }
  }
}     

# Call the function and collect any
# non-terminating errors in variable $errs.
# IMPORTANT: Pass the variable name *without the $*.
Invoke-BackupBDEKeys -ErrorAction SilentlyContinue -ErrorVariable errs

# If $errs is an empty collection, no errors occurred.
if (-not $errs) {

  "No errors occurred"
  # ... 
}
else {
  "At least one error occurred during the backup of BDE keys:`n$errs"
  # ...
}

Here's a minimal example, which uses a script block in lieu of a function:

& {
  [CmdletBinding()] param() Get-Item NoSuchFile 
} -ErrorVariable errs -ErrorAction SilentlyContinue
"Errors collected:`n$errs"

Output:

Errors collected:
Cannot find path 'C:\Users\jdoe\NoSuchFile' because it does not exist.