Access caller variables from threads (PS)

In PowerShell, threads getting executed in separate run spaces. To access variables from the caller scope, you have to use the ‘using’ scope modifier.

---

function DownloadFiles([string]$WebstoreUrl, [string]$FileList)
{
    Get-Content $FileList | ForEach-Object -Parallel {
        Invoke-WebRequest "$using:WebstoreUrl/$_" -OutFile $_
    } -ThrottleLimit 10
}

---

For safety reasons, you can’t mutate these variables directly. There is a workaround by assigning it to another variable, but better synchronize the access.

function DownloadFiles([string]$WebstoreUrl, [string]$FileList)
{
    $NoError = $true
    $SyncObject = [System.Object]::new()

    Get-Content $FileList | ForEach-Object -Parallel {
        try {
            Invoke-WebRequest "$using:WebstoreUrl/$_" -OutFile $_
        } catch {
            [System.Threading.Monitor]::Enter($SyncObject)
            try {
                Write-Output "Download error for $using:WebstoreUrl/$_"
                $NoError = $using:NoError
                $NoError = $false
            } finally {
                [System.Threading.Monitor]::Exit($SyncObject)
            }
        }
    } -ThrottleLimit 10

    return $NoError
}