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, Nov 24 '09 at 23:00

score 11 · Accepted Answer · answered Mar 29 '09 at 22:09

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
}

score 1 · Answer 2 · answered Oct 20 '09 at 23:43

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 '09 at 23:43

Ludovic Chabant

1,513
11
12

score 0 · Answer 3 · edited May 23 '17 at 11:59

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 '17 at 11:59

Community

1
1

answered Oct 19 '12 at 07:07

ya23

14,226
9
46
43

1

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. – Roman Kuzmin Oct 19 '12 at 07:16
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. – ya23 Oct 19 '12 at 08:07

score 0 · Answer 4 · answered Dec 05 '20 at 07:31

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()

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

4 Answers4

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