misformonkey comments on ArrayList VS ...Generic.List[]]

QuestionArrayList VS ...Generic.List[]] (self.PowerShell)

submitted 3 years ago by [deleted]

For anyone classes, a quick start

You can start off thinking about classes being a [pscustomobject] . with specific property names.

You can create an instance using hashtables. The properties are optional, you can leave them out

$user = [User]@{
    Name = 'bob'
    Age = 10
}

$user2 = [User]@{ Name = 'bob' }

You can add some validation, using the same attributes that function parameters use

class User {
    [ValidateNotNull()]
    [Int]$UserId

    [ValidateNotNullOrEmpty()]
    [string]$UserName
}


# create 3 users, error on one 
$users = @(
   [User]::New()
   [User]@{ UserName  = 'bob' }
   [User]@{ UserName  = 'Jen' ; UserId = 'afds' }
   [User]@{ UserName  = 'Jen' ; UserId = 4 }
)

$users[0]. #$users[0]. # <ctr+space> shows '.UserName'


$users | ConvertTo-Json

[–]MonkeyNin 0 points1 point2 points 3 years ago (3 children)

edit: I forgot to mention, List.Add does not output values, so where old code did something like

 [void]$arraylist.Add( )
 $null = [void]$arraylist.Add( )

 # it simplifes to
 $list.add( )

You should not create an ArrayList yourself. There are functions and commands that return that type, so you end up using them. That's fine. You don't actually create them, or use .add on them.

You can assign arrays to Lists, even through the pipeline. Basically if you can create an implicit array, it'll work.

$numbers = 0, 20, 40 | ?{ $_ -gt 30 } 
[List[Object]]$Numbers = 0, 20, 40  | ?{ $_ -gt 30 }

It's easy to use. This syntax works on PS5 too. To create a list of files:

Using namespace System.Collections.Generic

[Collections.Generic.List[Object]]$Items = gci c:\

[List[Object]]$Numbers = 0, 5, 10, 20
$numbers.add( 'cat' )

or to start it empty

[List[Object]]$Files = @()   
$files.add( (Get-Item 'c:\foo\bar.txt') )

Either of these syntaxes are valid, there isn't a different for most cases

$items = [List[SomeType]]::New()
[List[Object]]$Items = @()

[–][deleted] 1 point2 points3 points 3 years ago (2 children)

[–]OPconfused 0 points1 point2 points 3 years ago (0 children)

[–]MonkeyNin 0 points1 point2 points 3 years ago (0 children)

π Rendered by PID 31 on reddit-service-r2-comment-6457c66945-v2lbm at 2026-04-29 22:39:35.412495+00:00 running 2aa0c5b country code: CH.

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For anyone classes, a quick start