Running ForEach-Object -Parallel, data missing from export - powershell

Question

I have some working code that basically queries 2 different Graph API endpoints, then searches for a match in the User Principal Name column, and inserts the extension_335d4df9847945fbaa472c8b8fbb5d75_employeeNumber column and values to the exported csv (Thanks to the user #PMental for this solution) This column derives from attribute that was recently extended from our on premises AD.
This code works perfectly fine, however if I try to parallelize it, I get no results in the extension_335d4df9847945fbaa472c8b8fbb5d75_employeeNumber column.
Is this because once it is being parallelized, I'm not able to share variables between the parallel processes? If so, how on earth do I accomplish this?
Code below - if you remove the -Parallel, it works fine:
$graphApiUri = "https://graph.microsoft.com/v1.0/reports/getOffice365ActiveUserDetail(period='D90')"
$Uri = "https://graph.microsoft.com/v1.0/users?`$select=userPrincipalName,extension_335d4df9847945fbaa472c8b8fbb5d75_employeeNumber"
$O365Report = Invoke-RestMethod -Method Get -Uri $graphApiUri -Headers $headerParams | ConvertFrom-Csv
# If the result is more than 999, we need to read the #odata.nextLink to show more than one side of users
$UserDetails = while (-not [string]::IsNullOrEmpty($uri)) {
# API Call
$apiCall = try {
Invoke-RestMethod -Headers $headerParams -Uri $uri -Method Get
}
catch {
$errorMessage = $_.ErrorDetails.Message | ConvertFrom-Json
}
$uri = $null
if ($apiCall) {
# Check if any data is left
$uri = $apiCall.'#odata.nextLink'
$apiCall
}
}
Write-Output "Matching UPN to employeeNumber..."
$O365Report | ForEach-Object -Parallel {
$CurrentEmpNumber = $UserDetails.value |
Where-Object userPrincipalName -eq $_.'User Principal Name' |
Select-Object -ExpandProperty extension_335d4df9847945fbaa472c8b8fbb5d75_employeeNumber -ErrorAction SilentlyContinue
$_ | Add-Member -MemberType NoteProperty -Name extension_335d4df9847945fbaa472c8b8fbb5d75_employeeNumber -Value $CurrentEmpNumber
}
$O365Report | Export-Csv $ReportCSV -NoTypeInformation
Write-Output "Report saved to $ReportCSV."

Answer 1

When inside of a ForEach-Object -Parallel script block, and you are trying to reference variables which were created outside of it, you need to preface the variable name with using: so it would be $using:UserDetails
Examples:
Returns nothing because $test isn't accessible within the scope of the parallel script block:
$test = 1;
0..5 | % -Parallel { $test; };
Returns the value of $test five times because by using $using:test you are now able to see its value:
$test = 1;
0..5 | % -Parallel { $using:test; };
From documenation:
https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/foreach-object?view=powershell-7.1
The ForEach-Object -Parallel parameter set runs script blocks in parallel on separate process threads. The $using: keyword allows passing variable references from the cmdlet invocation thread to each running script block thread. Since the script blocks run in different threads, the object variables passed by reference must be used safely. Generally it is safe to read from referenced objects that don't change. But if the object state is being modified then you must used thread safe objects, such as .Net System.Collection.Concurrent types (See Example 11).
Personal note:
I would also recommend using -ThrottleLimit to limit its max degrees of paralellism. The default is 5, but you may want more or less than that depending on testing.