Yes, you create an empty array then use it to initialize the variable. Two other ways (System not needed as its namespace is already loaded): [Collections.ArrayList]$list = @() $list = [Collections.ArrayList]::new() New-Object is useful when you need -ComObject, or to dynamically create a type (typename stored as a string that you may not know when writing). It is the 'older' method before powershell had classes built-in. Also, if you create an 'untyped' variable from a cast, you can change its type later on. The first example I posted enforces the type for that variable, it can only be an arraylist. I recommend using the generic types where you can specify the type: [Collections.Generic.List[String]]$list = @() $list = [Collections.Generic.List[Int]]::new() Also save some typing and use a namespace: using namespace System.Collections.Generic [List[String]]$list = @() $list = [List[Int]]::new() And for speed (enumeration access), if the items are unique, try a HashSet: using namespace System.Collections.Generic [HashSet[String]]$list = @() $list = [HashSet[Int]]::new() You should be measuring speed on insertion/removal and access for real world speed tests. And you'll definitely see different results when you go past milestones like 10, 1000, 10000 items (generics are tailored to large datasets). Answer from chris-a5 on reddit.com
Vexx32
vexx32.github.io › 2020 › 02 › 15 › Building-Arrays-Collections
Building Arrays and Collections in PowerShell
Partially as a result of that, arrays (and more specifically object[]) are the backbone of PowerShell scripting. In the vast majority of cases, you're going to be handling arrays unless you deliberately choose to use another type of collection to handle output.
Practical 365
practical365.com › home › powershell › practical powershell: crafting collections
Practical PowerShell: Crafting Collections | Practical365
June 3, 2025 - A collection is a set of zero, one, or more objects. Within PowerShell, there are two common collection types: arrays and hash tables. However, you can leverage other collection types because PowerShell is built on the .NET Framework.
Videos
04:21
PowerShell Tutorial: Deep dive into Array Collections. - YouTube
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Grant Site Collection Admin SCA for support team. PowerShell demo ...
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Mastering PowerShell Dynamic Arrays - YouTube
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PowerShell Arrays - YouTube
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PowerShell 7 Tutorial 7: Introduction to Array and ArrayList - YouTube
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PowerShell 7 Tutorials for Beginners #3 : Array & ArrayList - YouTube
Medium
medium.com › @oakhelloworld › powershell-collection-968226024f38
Powershell: Collection. PowerShell provides a variety of data… | by Oak HelloWorld | Medium
September 28, 2023 - Powershell: Collection PowerShell provides a variety of data structures and collections for working with data in your scripts. Here are some common collection types in PowerShell: 1. Arrays: Arrays …
Microsoft Learn
learn.microsoft.com › en-us › powershell › scripting › lang-spec › chapter-04
Types - PowerShell | Microsoft Learn
Hashtable elements are stored in an object of type DictionaryEntry, and the collections returned by Keys and Values have type ICollection. Type xml implements the W3C Document Object Model (DOM) Level 1 Core and the Core DOM Level 2. The DOM is an in-memory (cache) tree representation of an XML document and enables the navigation and editing of this document. This type supports the subscript operator [] (§7.1.4.4). In PowerShell, xml maps to System.Xml.XmlDocument.
Red Gate Software
red-gate.com › home › powershell one-liners: collections, hashtables, arrays and strings
PowerShell One-Liners: Collections, Hashtables, Arrays and Strings | Simple Talk
January 23, 2019 - This is a multi-part series of PowerShell reference charts. Here you will details of the two fundamental data structures of PowerShell: the collection (array) and the hash table (dictionary), examining everything from creating, accessing, iterating, ordering, and selecting.
Reddit
reddit.com › r/powershell › two ways of creating a new system.collections.arraylist object, what's the difference?
r/PowerShell on Reddit: Two ways of creating a new System.Collections.ArrayList object, what's the difference?
April 7, 2023 -
I've come across two ways of creating a new System.Collections.ArrayList object:
$arrA = New-Object System.Collections.ArrayList $arrB = [System.Collections.ArrayList]@()
I have two questions:
-
for arrB, if I'm reading this right, @() is creating an empty array and then it's being cast into [System.Collections.ArrayList]?
-
Working with the created object is the same for me either way, but are there any differences I may be missing?
I made a quick test and casting was faster, but the difference is only noticeable when creating new arrays reaches the hundreds of thousands to millions (tested on an old A10-7870k). Doesn't really matter when I'm creating one array, but I thought it was mildly interesting.
$max = 100000
$ticksStart = (Get-Date).Ticks
for ($i = 0; $i -lt $max; $i++)
{
$arrA = New-Object System.Collections.ArrayList
}
$ticksEnd = (Get-Date).Ticks
Write-Host 'arrA (ticks) : ' ($ticksEnd - $ticksStart)
$ticksStart = (Get-Date).Ticks
for ($i = 0; $i -lt $max; $i++)
{
$arrB = [System.Collections.ArrayList]@()
}
$ticksEnd = (Get-Date).Ticks
Write-Host 'arrB (ticks) : ' ($ticksEnd - $ticksStart)
arrA (ticks) : 64310000
arrB (ticks) : 3170000 Top answer 1 of 2
28
Yes, you create an empty array then use it to initialize the variable. Two other ways (System not needed as its namespace is already loaded): [Collections.ArrayList]$list = @() $list = [Collections.ArrayList]::new() New-Object is useful when you need -ComObject, or to dynamically create a type (typename stored as a string that you may not know when writing). It is the 'older' method before powershell had classes built-in. Also, if you create an 'untyped' variable from a cast, you can change its type later on. The first example I posted enforces the type for that variable, it can only be an arraylist. I recommend using the generic types where you can specify the type: [Collections.Generic.List[String]]$list = @() $list = [Collections.Generic.List[Int]]::new() Also save some typing and use a namespace: using namespace System.Collections.Generic [List[String]]$list = @() $list = [List[Int]]::new() And for speed (enumeration access), if the items are unique, try a HashSet: using namespace System.Collections.Generic [HashSet[String]]$list = @() $list = [HashSet[Int]]::new() You should be measuring speed on insertion/removal and access for real world speed tests. And you'll definitely see different results when you go past milestones like 10, 1000, 10000 items (generics are tailored to large datasets).
2 of 2
10
Side note. Don’t use an array list if you know the underlying type. Use a generic list.
powershelldistrict
powershelldistrict.com › powershell-queue-collection
The powershell queue collection – powershelldistrict
Working with powershell, generally a simple array declared and used like this will generally be enough. ... Although this is trivial, and very often enough for our daily business (or in other words, we will always find a way to make it work). there are sometime some other (better?) ways to do things. The System.Collection.Queue is just like a regular array, but is has some additional convenient methods that allow us to always return the first element added to our queue.
Microsoft Learn
learn.microsoft.com › en-us › powershell › scripting › learn › deep-dives › everything-about-arrays
Everything you wanted to know about arrays - PowerShell | Microsoft Learn
March 24, 2025 - The foreach loop works well with collections. Using the syntax: foreach ( <variable> in <collection> ) ... I tend to forget about this one but it works well for simple operations. PowerShell allows you to call ForEach() on a collection.
Stack Overflow
stackoverflow.com › questions › 76964534 › how-to-write-to-powershell-collection
arrays - How to write to Powershell collection - Stack Overflow
I want to add output from below command to some sort of array or collection: Get-MpThreatDetection | Where-Object {$_.InitialDetectionTime -ge $time} It could produce output of multiple detections...
ScriptRunner
scriptrunner.com › blog-admin-architect › powershell-array-types
Exploring PowerShell Array Types: A Comprehensive Guide
May 22, 2025 - This example uses a [string[]]: a user can enter ten names, and if a number is accidentally added, PowerShell will automatically convert it to a string: # new empty string array [string[]]$names = @() # let user enter 10 names for ($x=1; $x-le10;$x++) { $names += Read-Host -Prompt "Enter $x. Name" } $names.Count · Whats most important about the last example is the reappearance of the dreaded += problem we previously discussed: how can you use a typed array that can dynamically grow or shrink? [System.Collections.ArrayList] is not typed.
Top answer 1 of 2
12
If you remove the array subexpression @(...) and just precede with a comma. The below code seems to work:
function TestReturn {
$returnList = New-Object System.Collections.Generic.List[string]
$returnList.Add('Testing, one, two')
return , $returnList
}
$testList = TestReturn
$testList.GetType().FullName
Note: technically this causes the return of [Object[]] with a single element that's of type [System.Collections.Generic.List[string]]. But again because of the implicit unrolling it sort of tricks PowerShell into typing as desired.
On your later point, the syntax [Type]$Var type constrains the variable. It's basically locking in the type for that variable. As such subsequent calls to .GetType() will return that type.
These issues are due to how PowerShell implicitly unrolls arrays on output. The typical solution, somewhat depending on the typing, is to precede the return with a , or ensure the array on the call side, either by type constraining the variable as shown in your questions, or wrapping or casting the return itself. The latter might look something like:
$testList = [System.Collections.Generic.List[string]]TestReturn
$testList.GetType().FullName
To ensure an array when a scalar return is possible and assuming you haven't preceded the return statement with , , you can use the array subexpression on the call side:
$testList = @( TestReturn )
$testList.GetType().FullName
I believe this answer deals with a similar issue
2 of 2
4
In addition to Steven's very helpful answer, you also have the option to use the [CmdletBinding()] attribute and then just call $PSCmdlet.WriteObject. By default it will preserve the type.
function Test-ListOutput {
[CmdletBinding()]
Param ()
Process {
$List = New-Object -TypeName System.Collections.Generic.List[System.String]
$List.Add("This is a string")
$PSCmdlet.WriteObject($List)
}
}
$List = Test-ListOutput
$List.GetType()
$List.GetType().FullName
For an array, you should specify the type.
function Test-ArrayOutput {
[CmdletBinding()]
Param ()
Process {
[Int32[]]$IntArray = 1..5
$PSCmdlet.WriteObject($IntArray)
}
}
$Arr = Test-ArrayOutput
$Arr.GetType()
$Arr.GetType().FullName
By default the behaviour of PSCmdlet.WriteObject() is to not enumerate the collection ($false). If you set the value to $true you can see the behaviour in the Pipeline.
function Test-ListOutput {
[CmdletBinding()]
Param ()
Process {
$List = New-Object -TypeName System.Collections.Generic.List[System.String]
$List.Add("This is a string")
$List.Add("This is another string")
$List.Add("This is the final string")
$PSCmdlet.WriteObject($List, $true)
}
}
Test-ListOutput | % { $_.GetType() }
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True String System.Object
True True String System.Object
True True String System.Object
(Test-ListOutput).GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True Object[] System.Array
# No boolean set defaults to $false
function Test-ListOutput {
[CmdletBinding()]
Param ()
Process {
$List = New-Object -TypeName System.Collections.Generic.List[System.String]
$List.Add("This is a string")
$List.Add("This is another string")
$List.Add("This is the final string")
$PSCmdlet.WriteObject($List)
}
}
Test-ListOutput | % { $_.GetType() }
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True List`1 System.Object
(Test-ListOutput).GetType()
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
True True List`1 System.Object
Just wanted to add some information on what I usually use in functions and how to control the behaviour.
IDERA
idera.com › home › using efficient lists in powershell
Using Efficient Lists in PowerShell | IDERA
April 22, 2025 - Learn how to handle dynamic lists efficiently in PowerShell using System. Collections.ArrayList and generic lists instead of default object.
| How
pipe.how › new-hashtable
PowerShell Collections: Hashtable | How
October 27, 2020 - Hashtables aren’t structures exclude to PowerShell, you can find them in many languages and places. The thing that differentiates them from traditional arrays and lists is that they’re based on keys and values. If you’re familiar with dictionary data types, the hashtable is one of them.
Adam the Automator
adamtheautomator.com › powershell-array
PowerShell Arrays, ArrayLists & Collections: Best Practices
Using a PowerShell ArrayList is also a way in which you can store a list of items with PowerShell. The ArrayList class is part of the System.Collections namespace within .NET.
Published April 22, 2025
EDUCBA
educba.com › home › data science › data science tutorials › powershell tutorial › powershell array of strings
PowerShell Array of Strings | Guide to PowerShell Array of Strings
May 19, 2023 - PowerShell array of strings is the collection of string objects that is multiple strings residing into the same group, which can be declared using String [], @(), or the ArrayList and can be used in various ways like functions, in the file content, as a variable and can perform the multiple operations on the string array like Split, Join, trim, ToUpper, ToLower, etc.
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Jonathanmedd
jonathanmedd.net › 2014 › 01 › adding-and-removing-items-from-a-powershell-array.html
Adding and Removing Items from a PowerShell Array - Jonathan Medd's Blog
Taking your initial array you can convert it to a System.Collections.ObjectModel.Collection`1 like this, which may be easier to remember than using the full type name of either a collection or array list:
Dcrich
blog.dcrich.net › post › 2022 › powershell-journeyman-generic-collections
PowerShell Journeyman - Generic Collections · Of Computers and Mice
The generic list will feel quite familiar and is generally considered the best collection to use for operations due to its superior performance. There are other arrays in PowerShell like @() and [Collections.ArrayList]. PowerShell is always evolving! But one of the team’s design goals was not to break existing code where possible.
SharePoint Diary
sharepointdiary.com › sharepoint diary › powershell › powershell tutorials › powershell arraylist – a beginners guide!
PowerShell ArrayList - A Beginners Guide! - SharePoint Diary
September 30, 2025 - However, standard PowerShell arrays have limitations – they are fixed-sized and don’t provide many helpful methods to manipulate the array contents. This is where PowerShell ArrayLists come in. The ArrayList is one of the most useful collection types in PowerShell.
powershelldistrict
powershelldistrict.com › powershell-stack-collection
The PowerShell Stack collection – powershelldistrict
August 13, 2023 - I didn’t knew what it was, so I looked it up, read about it, and applied it to powershell immediatley. In this article I will go through what the System.collections.stack actually is, and in what cases it should be used. I’ll explain of course how to use in Powershell, and showcase some examples where one might want to use them.
Varonis
varonis.com › blog › powershell-array
PowerShell Array Guide: How to Use and Create
June 9, 2022 - As we’ve said above, adding items to arrays can be a hassle. However, there is a different type of collection – ArrayList – that handles this more elegantly.