ThreadJobs has access to the same environment as it was started in.
But normally, PowerShell will respond with a syntax error when trying to change variables from the parent level.
The documentation MS Learn - about_Thread_Jobs has som insights but nothing I could find useful.
The example below illustrates the issue when trying to use plain PowerShell variables.
[Array]$Numbers = @()
foreach ($i in 0..11) {
$Jobs = Start-ThreadJob {
$Using:Numbers += $using:i
}
}
$Jobs | Wait-Job | Remove-Job
$Numbers
ParserError:
Line |
6 | $Using:Numbers += $using:i
| ~~~~~~~~~~~~~~~
| The assignment expression is not valid. The input to an assignment operator must
| be an object that is able to accept assignments, such as a variable or a property.
As the threads are run in parallell, there has to be some kind of method that prevents the parent object from getting corrupted or a ThreadJob from failing if two or more threads tries to perform operations at the exact same time on the object.
After wrestling several days with the concept of thread safe execution (and getting great help with the patience of Santiago Squarzon and others, my own conclusion is:
All the operation in the thread has to be made thread safe (hence the name).
Using:
if the objects are "plain" unless you can guarantee the value has been lockedIn .Net, I found two thread safe classes you can work with
(There is one class name available per T-reference.)
None of the classes has a thread safe method for incrementing values.
So a thread safe ThreadJob in PowerShell 7.x, only adding new items to parent objects, might look like this
$SafeNumbers = [System.Collections.Concurrent.ConcurrentBag[object]]::new()
foreach ($i in 0..11) {
$Thread = Start-ThreadJob {
($Using:SafeNumbers).Add($Using:i)
}
}
Get-Job | Wait-Job | Remove-Job
$SafeNumbers.ToArray()
11
10
9
8
7
6
5
4
3
2
1
0
The order of the output is of course not guaranteed.