PowerShell Weekly
PowerShell Weekly

PoshBytes: Update-List

Matthew Dowst

PoshBytes: Update-List

The often forgotten, but always useful Update-List.

This post is a companion for the video embedded below. Scroll down to see the code from the video.

Create Sample Objects

Define a helper function that returns a test server object with a Tags list, then create two copies for side-by-side comparisons.

Function Get-TestSrv {
    param($n = 1)
    [pscustomobject]@{
        Name = "SRV0$n"
        Tags = [Collections.Generic.List[string]]('dev', 'windows')
    }
}
$clean = Get-TestSrv
$clunky = Get-TestSrv
$clean
$clunky

Add Items the Manual Way

Use .Add() directly on the list to append new tags.

$clunky.Tags.Add('prod')
$clunky.Tags.Add('sql')
$clunky

Add Items with Update-List

Add multiple items to the list property in a single, declarative call.

$clean | Update-List -Property Tags -Add 'prod', 'sql'

Remove Items the Manual Way

Remove specific tags by calling .Remove() on the list for each item.

$clunky.Tags.Remove('dev')
$clunky.Tags.Remove('sql')
$clunky

Remove Items with Update-List

Remove multiple tags from the list property in one command.

$clean | Update-List -Property Tags -Remove 'dev', 'sql'

Mix Remove and Add the Manual Way

Manually remove one tag and add another to reshape the list.

$clunky.Tags.Remove('windows')
$clunky.Tags.Add('linux')
$clunky

Mix Remove and Add with Update-List

Perform both removal and addition in a single, readable command.

$clean | Update-List -Property Tags -Remove 'windows' -Add 'linux'

Replace the Entire List Manually

Overwrite the property with a brand new list instance containing the target values.

$clunky.Tags = [Collections.Generic.List[string]]('critical', 'linux', 'sql')
$clunky

Replace the Entire List with Update-List

Use -Replace to set the list property to an exact set of values in one step.

$clean | Update-List -Property Tags -Replace 'critical', 'linux', 'sql'

Create Multiple Servers

Prepare two sets of server objects for batch updates and comparisons.

$cleanServers = 1..3 | ForEach-Object { Get-TestSrv $_ }
$clunkyServers = 1..3 | ForEach-Object { Get-TestSrv $_ }
$clunkyServers

Batch Update Manually with a Loop

Iterate all objects and call .Add() on each list property.

$clunkyServers | ForEach-Object {
    $_.Tags.Add('monitored')
}
$clunkyServers

Batch Update with Update-List

Pipe the collection into Update-List to modify every object’s list property in one pipeline.

$cleanServers | Update-List -Property Tags -Add 'monitored'

Pitfall of Using + on a List Property

Using + forces an array operation that changes the property type away from a list.

$clunky.Tags = $clunky.Tags + 'oops'
$clunky.Tags.GetType().FullName

Manual Repair Back to List

Rebuild the list by copying items into a new List[string] and assign it back.

$tmp = [Collections.Generic.List[string]]::new()
$tmp.AddRange([string[]]$clunky.Tags)
$tmp.Add('fixed')
$clunky.Tags = $tmp
$clunky.Tags.GetType().FullName

Update-List Preserves the List Type

Adding with Update-List keeps the property as a list without converting it to an array.

$clean | Update-List -Property Tags -Add 'woot'
$clean.Tags.GetType().FullName

Capturing a Delta and Applying It

Use Update-List to build a reusable delta object, then apply it to an existing list.

$delta = Update-List -Add 'gpu' -Remove 'linux'
([pslistmodifier]$delta).ApplyTo($clean.Tags)
$clean.Tags
  • Update-List edits list properties in place: -Add, -Remove, -Replace.
  • Pipeline-friendly; add and remove together in one call.
  • Can emit a change set [pslistmodifier] to apply later.
  • Avoids verbose read–modify–reassign patterns with = / +=.
  • Reintroduced in PowerShell 7+;
  • Works with any property that supports IList.

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