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?

Answer 1

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.

Answer 2

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