How to iterate through a collection of items in a Parallel Foreach loop in PowerShell?

That’s a mouthful, right? But yeah – this article shows and tells how to do a parallel foreach loop in PowerShell, and how to do it properly!

Well, at least probably. This is how I did it anyway. But that’s enough chitchat, let’s get to it!

Background

In PowerShell, you can iterate through objects with a ForEach-Object loop like this:

# Create an array of integers
$items = 1..10
# Iterate through the items in the array
$items | ForEach-Object {
# Perform some operation on each item
$_ * 2
}

This code little script will loop through your integers one by one. And that’s nice and all – but what if you want to be fast, furious and utterly reckless?

Well, in comes the -Parallel switch!

What is a “Parallel” Foreach loop in PowerShell anyway?

By default, Foreach-Object processes items sequentially, meaning it executes the operation on one item at a time, waiting for each item to finish before moving on to the next.

A parallel ForEach-Object loop, on the other hand, enables you to execute the operations on multiple items simultaneously, taking advantage of the available processing power to improve efficiency. It allows for parallel execution of the script blocks within the loop, which can be beneficial when dealing with computationally intensive tasks or when you have independent operations that don’t rely on each other’s results.

PowerShell provides the ForEach-Object -Parallel parameter to enable parallel processing. Here’s an example of how it can be used:

$items = 1..10

$items | ForEach-Object -Parallel {
    # Perform some operation on each item
    $_ * 2
}

In the above example, the ForEach-Object -Parallel loop processes each item in parallel, executing the script block for each item simultaneously. The result is a new collection containing the doubled values of the original items.

It’s important to note that parallel execution gives 0 guarantees the order of the output elements to be the same as the input collection. Additionally, parallel execution introduces concurrency, which can have implications depending on the nature of the operations being performed. Therefore, it’s essential to consider potential race conditions or conflicts that may arise when modifying shared resources within a parallel loop.

Parallel foreach loops can significantly improve performance in certain scenarios, but it’s important to evaluate the specific use case and determine if parallel execution is appropriate and safe for your particular scenario.

Solution

Let’s go through a real-life example of a parallel foreach loop in PowerShell! In my case, I’m getting a bunch of AAD users, iterating through them to perform some magic on each user, and updating progress after each execution.

A copy-pasteable script snippet

You didn’t think I’d only share you the concepts and not the actual code, did you? :)

Anyway – the pattern ends up looking somewhat like this:

# Get a collection of objects - in my case, I'm getting all users on a tenant
# It's actually a bit more complicated than this, but
# that's beyond the scope of this article
$users = Get-PnpUser 

$total = $users.Count

# We then pass the index of items to Foreach-Object
# Otherwise we can't get the "index" of each item in the
# collection

# This is done by instantiating an array of ints between
# "0" and "total number of items -1"
0..$($total-1) | Foreach-Object -Parallel {

  # Then we'll grab the $users from outside the loop
  # by calling it with using -keyword
  $usersInternal = $using:users

  # Then we can grab the user from the internal collection
  # (Because you can't access the "external" array
  # with the loop iterator $_)
  $user = $usersInternal[$_]

  # Now we can do whatever we want with the user

  # And of course, update progress like this:
  $percentage = $_ / $using:total * 100

  Write-Progress -Activity "Search in Progress" -Status "$($percentage) % Complete:" -PercentComplete $percentage
}

And now comes the question – did I have to figure out this convoluted way of looping through items just to update the progress properly? Was that the only reason why I would go through all this?

I guess we’ll never know :)

• Author
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Antti K. Koskela

Developer / Speaker / Consultant at Koskila / Precio Fishbone / Norppandalotti Software Co / Alter - Experience Ideas Ltd

Antti Koskela is a proud digital native nomadic millennial full stack developer (is that enough funny buzzwords? That's definitely enough funny buzzwords!), who works as Solutions Architect for Precio Fishbone, building delightful Digital Workplaces.

He's been a developer from 2004 (starting with PHP and Java), and he's been working on .NET projects, Azure, Office 365, SharePoint and a lot of other stuff. He's also Microsoft MVP for Azure.

This is his personal professional (e.g. professional, but definitely personal) blog.

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