PowerShell converts values returned from function. How to avoid this?

Question

I am trying to return List< T> from PowerShell function, but get one of:

null - for empty list
System.Int32 - for list with one element
System.Object[] - for list with more elements

Code:

function CreateClrList
{
    $list = New-Object "System.Collections.Generic.List``1[System.Int32]"
    $list.Add(3)
    $list
}

Write-Host (CreateClrList).GetType()

Better title: converts IEnumerable collections into object arrays. — Peter Seale
– Peter Seale, Commented Nov 24, 2009 at 23:00

user83286 · Accepted Answer · 2009-03-29 22:09:38Z

11

Yeah, powershell unrolls all collections. One solution is to return a collection containing the real collection, using the unary comma:

function CreateClrList
{
    $list = New-Object "System.Collections.Generic.List``1[System.Int32]"
    $list.Add(3)
    ,$list
}

answered Mar 29, 2009 at 22:09

user83286

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Comments

Ludovic Chabant · Accepted Answer · 2009-10-20 23:43:45Z

1

Note that most of the time, you want Powershell to unroll enumerable types so that piped commands execute faster and with earlier user feedback, since commands down the pipe can start processing the first items and give output.

answered Oct 20, 2009 at 23:43

Ludovic Chabant

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Comments

Community · Accepted Answer · 2017-05-23 11:59:53Z

0

If you need to return a list of ints, use jachymko's solution.

Otherwise, if you do not particularly care whether what you get is a list or array, but just want to iterate through result, you can wrap result in @() while enumerating, e.g.

$fooList = CreateClrList
foreach ($foo in @($fooList))
{
    ...
}

That would cause @($fooList) to be of array type - either an empty array, array with one element or array with multiple elements.

edited May 23, 2017 at 11:59

CommunityBot

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answered Oct 19, 2012 at 7:07

ya23

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2 Comments

Roman Kuzmin Over a year ago

In that case adding items to the internal list would not be used, int values could be output right away. I think the author wanted the list to be returned exactly.

ya23 Over a year ago

Good point. You're probably right - that is what he said he wanted. However, it may not be the thing he ultimately wants - just providing alternative option.

Jan David Narkiewicz · Accepted Answer · 2020-12-05 07:31:35Z

0

PowerShell 5.0 added support for classes and hence methods where the return type can be specified. To control the type returned, use a class with a static method:

class ListManager {
    static [System.Collections.Generic.List[int]] Create() {
        [System.Collections.Generic.List[int]] $list =
            [System.Collections.Generic.List[int]]::new()

        $list.Add(3)

        return $list
     }
}

[ListManager]::Create().GetType()

answered Dec 5, 2020 at 7:31

Jan David Narkiewicz

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