PowerShell ForEach-Object -Parallel

ForEach-Object -Parallel in PowerShell 7 runs each iteration in its own runspace at the same time. One switch turns a serial loop into a concurrent one.

Lesson 2 min read Windows PowerShell 5.1 and PowerShell 7

Documentation: Microsoft Learn

Written and tested by Dhrub Bharali · Updated 02 October 2026

Overview

The rule that trips everyone: each iteration is a separate runspace, so your variables are not there. $prefix from the calling scope is empty inside the block. Prefix it with $using: to bring it in, so the string becomes $using:prefix followed by a dash and $_. Miss that and you get blanks or nulls, not an error.

-ThrottleLimit controls how many run concurrently and defaults to 5. Raise it for network-bound work where you are waiting on responses; leave it low for anything CPU-bound or hitting a single machine.

Parallel is not automatically faster. Each runspace has setup cost, so for a fast operation over a small collection the serial loop wins. It pays off when each iteration takes real time, particularly waiting on the network.

Order is not preserved. Results arrive as they complete, so sort afterwards if order matters.

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$prefix = 'Server'
1 .. 4 | ForEach-Object {
[pscustomobject]@{
    Index = $_
    Name = "$prefix-$_"
  } }

Stage 1$prefixPositional values or no named parameters
Stage 2ForEach-ObjectPositional values or no named parameters

An example to inspect

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$prefix = 'Server'
1 .. 4 | ForEach-Object {
[pscustomobject]@{
    Index = $_
    Name = "$prefix-$_"
  } }

What happens

This is the serial version, which works because $prefix is in scope. Add -Parallel and the Name column loses the prefix, because $prefix does not exist in the child runspace. The fix is $using:prefix in place of $prefix. That one substitution is the difference between correct output and silently blank output.

Worth remembering

PowerShell 7 only. Each iteration gets its own runspace.
Outer variables need $using: or they are empty.
-ThrottleLimit defaults to 5.
Results come back out of order.

Primary documentation

ForEach-Object reference

Try it

Illustrative output · values vary by environment

1PS> $prefix = 'Server'

2PS>

3PS> 1 .. 4 | ForEach-Object {

4PS> [pscustomobject]@{

5PS> Index = $_

6PS> Name = "$prefix-$_"

7PS> } }

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$prefix = 'Server'

1 .. 4 | ForEach-Object {
  [pscustomobject]@{
    Index = $_
    Name = "$prefix-$_"
  } }

Ready to inspect.

The serial form of a loop, ready to compare against -Parallel.

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