How to add a custom property to a PowerShell array?

Question

Say I have a PowerShell array $Sessions = @() which I am going to fill with PSCustomObjects. How can I add a custom property to the array itself? E.g. so I can have $Sessions.Count which is built-in and $Sessions.Active which I want to set to the active session count.

I know that I can add properties to PSCustomObjects (in a dirty way) with

$MyCustomObject = "" | Select-Object Machine, UserName, SessionTime

but though doing so on an array would not result in the property being added.

So how can I achieve my goal? Is there any way to create a custom array?

Just use Add-Member -InputObject $sessions ...? That is what it is for — Matt
– Matt, Commented Sep 11, 2017 at 11:51
Beware, though, that appending to that array ($sessions += ...) will replace the array, thus removing the additional property. A safer approach would be a custom object with distinct properties for array and state. With that said, depending on what you want to use this for it might be more appropriate to use different list variables $activeSessions/$inactiveSessions instead of adding a property to the list, or to add an Active property to the individual session objects. — Ansgar Wiechers
– Ansgar Wiechers, Commented Sep 11, 2017 at 12:21
@Matt if you only would've postet as an answer... I actually abused Select-Object out of laziness which was what kept me from my solution. Thanks. @AnsgarWiechers thanks for the hint about +=, saved me from misplacing the Add-Member call. — FatalMerlin
– FatalMerlin, Commented Sep 11, 2017 at 12:27
I forgot to mention the Select you did is a perfectly acceptable method of creating a blank slate object. To each there own in this regard. The only setback from your method is in how basic it can be for things like types and complex properties. Still nothing wrong with it. — Matt
– Matt, Commented Sep 11, 2017 at 14:56

Community · Accepted Answer · 2020-06-20 09:12:55Z

The answer to your question as stated would be to just use Add-Member on the array object.

Add-Member -InputObject $sessions -MemberType NoteProperty -Name "State" -Value "Fabulous"

Adding a property to each element after you created the object is similar.

$sessions | ForEach-Object{
    $_ | Add-Member -MemberType NoteProperty -Name "State" -Value "Fabulous"
}

This of course comes with a warning (that I forgot about). From comments

Beware, though, that appending to that array ($sessions += ...) will replace the array, thus removing the additional property.

^{Ansgar Wiechers}

Depending on your use case there are other options to get you want you want. You can save array elements into distinct variables:

# Check the current object state
$state = $object.Property .....

# Add to the appropriate array.
if($state -eq "Active"){
    $activeSessions += $object
} else {
    $inactiveSessions += $object
}

Or you could still store your state property and post process with Where-Object as required:

# Process each inactive session
$sessions | Where-Object{$_.State -eq "Active"} | ForEach-Object{}

To avoid the destroying / recreating array issue, which can be a performance hog, you could also use an array list instead.

$myArray = New-Object System.Collections.ArrayList
Add-Member -InputObject $myArray -MemberType ScriptMethod -Name "NeverTellMeTheOdds" -Value {
    $this | Where-Object{$_ % 2 -ne 0}
}

$myArray.AddRange(1..10)
$myArray.NeverTellMeTheOdds()

Notice that the array had its member added then we added its elements.

Mathias R. Jessen · Accepted Answer · 2017-09-11 13:00:11Z

3

As Matt commented, you can use the Add-Member on an enumerable type by supplying it as a positional argument to the -InputObject parameter.

To allow for resizing after adding the new property, use a generic List instead of @():

$list = [System.Collections.Generic.List[psobject]]::new()
$list.AddRange(@(
  [pscustomobject]@{SessionId = 1; Active = $true}
  [pscustomobject]@{SessionId = 2; Active = $false}
  [pscustomobject]@{SessionId = 3; Active = $true}
) -as [psobject[]])

Add-Member -InputObject $list -MemberType ScriptProperty -Name ActiveSessionCount -Value {
  return @($this |? Active -eq $true).Count
}

Now you can retrieve the active session count easily:

PS C:\> $list.ActiveSessionCount
2

answered Sep 11, 2017 at 13:00

Mathias R. Jessen

178k13 gold badges175 silver badges234 bronze badges

1 Comment

Matt Over a year ago

I suggested an arraylist. We think alike it seems.

Collectives™ on Stack Overflow

How to add a custom property to a PowerShell array?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related