How to add a custom property to a PowerShell array?

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.ArrayListAdd-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.

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.ActiveSessionCount2