How to pass ArrayList to functions and get results back in powershell

Question

I am trying to pass an array to functions and fill it, and then print the results outside of the functions.
But the first function doesn't recognize the array list object I am passing to it.

Main file:

. $funcFile
$myParam = "Hello World"  
$myObj = getMyObject $myParam
$myObj.myArrayList.Count   # This works (outputs 0)
myFunction2 ($myObj.myArrayList)
$myObj.myArrayList.Count   # This also works (outputs 0)

fncFile:

function getMyObject([String] $myParam) {
    $myObj = @{  
         "myArrayList" = (New-Object System.Collections.ArrayList)  
    }
    return $myObj
}

function myFunction2 ([System.Collections.ArrayList] $myArr){
    $myArr.Count  # This doesn't work (outputs nothing)
    if($myArr -eq $null) {
         Write-Host "Array List Param is null"   # This condition is FALSE - nothing is printed
    }
}

What am I doing wrong?
How can I use the same ArrayList in function2 and other inner functions?

I can not reproduce described behavior on my PC. What PowerShell version did you use to reproduce the issue with given code? — user4003407
– user4003407, Commented Feb 4, 2019 at 10:38
@Davis8988 after the edit it print 3 zeros. Seems to be working to me — Bakudan
– Bakudan, Commented Feb 4, 2019 at 12:11

Bakudan · Accepted Answer · 2019-02-04 09:05:04Z

1

If you want to pass a variable and modify it in function and use the result there are 2 ways:

Pass by value:

$arr = New-Object System.Collections.ArrayList
function FillObject([System.Collections.ArrayList]$array, [String] $myParam) {
    return $array.Add($myParam)
}
$arr = FillObject -array $arr -myParam "something"
$arr.Count

Pass by reference (what you asked about)

[System.Collections.Generic.List[String]]$lst = (New-Object System.Collections.Generic.List[String]])
function FillObject([System.Collections.Generic.List[String]][ref]$list,[String] $myParam) {
    $list.Add($myParam)
}
FillObject -list ([ref]$lst) -myParam "something"
$lst.Count

You have to add [ref] both in the function definition and when you pass the parameter. If this will help you - Powershell and C# rely on .NET, so their syntax is similar. C# way to use the ref:

int number = 1;
void Method(ref int refArgument)
{
    refArgument = refArgument + 44;
}
Method(ref number);

edited Feb 4, 2019 at 9:05

answered Feb 4, 2019 at 8:59

Bakudan

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2 Comments

Davis8988 Over a year ago

I tried what your wrote but it still doesn't work.. Besides, according to Microsoft documentation objects are passed by reference by default. Since ArrayList is an object, the passing of it is by reference in both of your examples if you include [ref] or not.

Bakudan Over a year ago

The thing is to be explicit about what you want. It helps in later stages of development.

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