Passing variables to runspaces and back - powershell

Question

so I have the following code:
#region Initialize stuff
$files = gci "C:\logs\*.log"
$result = [System.Collections.ArrayList]::Synchronized((New-Object System.Collections.ArrayList))
$RunspaceCollection = #()
$RunspacePool = [RunspaceFactory]::CreateRunspacePool(1,5)
$RunspacePool.Open()
$ScriptBlock = {
Param($file, $result)
$content = Get-Content $file.FullName -ReadCount 0
foreach ($line in $content) {
if ($line -match 'A002') {
[void]$result.Add($($line -replace ' +',","))
}}}
#endregion
#region Process Data
Foreach ($file in $files) {
$Powershell = [PowerShell]::Create().AddScript($ScriptBlock).AddArgument($file).AddArgument($result)
$Powershell.RunspacePool = $RunspacePool
[Collections.Arraylist]$RunspaceCollection += New-Object -TypeName PSObject -Property #{
Runspace = $PowerShell.BeginInvoke()
PowerShell = $PowerShell
}}
While($RunspaceCollection) {
Foreach ($Runspace in $RunspaceCollection.ToArray()) {
If ($Runspace.Runspace.IsCompleted) {
[void]$result.Add($Runspace.PowerShell.EndInvoke($Runspace.Runspace))
$Runspace.PowerShell.Dispose()
$RunspaceCollection.Remove($Runspace)
}}}
#endregion
#region Parse Data
$data = ConvertFrom-Csv -InputObject $result -Header "1","2","3","TimeIn","TimeOut","4","5","Dur"
foreach ($line in $data) {
if ($line.TimeIn -match "A002") { $TimeIn += [timespan]::Parse($line.Dur) }
else { $TimeOut += [timespan]::Parse($line.Dur) }}
#endregion
It works, but I don't completely understand how ;)
what is [System.Collections.ArrayList]::Synchronized((New-Object System.Collections.ArrayList)) and why does it work, while regular ArrayList doesn't? Why do I need this "Synchronized" and what it does? Could you please explain or point me to some materials about this? I can't seem to find anything relevant. Thank you!

Answer 1

To guarantee the thread safety of the ArrayList, all operations must
be done through this wrapper.
Enumerating through a collection is intrinsically not a thread-safe
procedure. Even when a collection is synchronized, other threads can
still modify the collection, which causes the enumerator to throw an
exception. To guarantee thread safety during enumeration, you can
either lock the collection during the entire enumeration or catch the
exceptions resulting from changes made by other threads.
Source:
https://msdn.microsoft.com/en-us/library/3azh197k(v=vs.110).aspx
This was found by googling
ArrayList Synchronized Method
It would be most beneficial for you to learn the fundamentals of object oriented script/programming so in the future you know exactly what to look for. There are many ways to implement .NET namespaces, classes, methods, and elements in a powershell host, [System.Collections.ArrayList]::Synchronized((New-Object System.Collections.ArrayList)) being one of them.