Adding to one element in array results in adding to all elements

Question

I have the following:

$p = New-Object PSObject -Property @{PIndex = New-Object System.Collections.ArrayList}
[System.Collections.ArrayList]$pa = @($p) * 5

I want the array $pa (containing the $p object) to be of fixed size. So far all looks good. But when I add elements to PIndex of one member of array $pa, it adds the same elements to all the other members of $pa. So, I do this:

$pa[0].PIndex.Add(2)

Lets check:

PS C:\Users> $pa[0].PIndex
2
PS C:\Users> $pa[1].PIndex
2
PS C:\Users> $pa[2].PIndex
2

And so on. Just want to add elements to PIndex array of $pa[0]. Why is it also adding to the other members ? Am I missing a syntax ?

Type looks fine:

PS C:\Users> $pa.GetType()

IsPublic IsSerial Name                                     BaseType                                                                            
-------- -------- ----                                     --------                                                                            
True     True     ArrayList                                System.Object

Thanks..

You're passing by reference instead of by value. When you edit one, you edit them all. — Maximilian Burszley
– Maximilian Burszley, Commented Jun 25, 2018 at 19:23
Reference/Value doesn't answer the question. Each member is distinct. I am pointing to a specific member. When I refer to $pa[0], I am not referring to $pa[1] or any other. Maybe the syntax.. — Sorizon
– Sorizon, Commented Jun 25, 2018 at 19:37
Each is not distinct. that is why changing one is changing all. You have essentially added one object to the arraylist five times. — EBGreen
– EBGreen, Commented Jun 25, 2018 at 19:39
Do this if you want to add five different objects: $pa = New-Object System.Collections.ArrayList;1..5 | %{$p = New-Object PSObject -Property @{PIndex = New-Object System.Collections.ArrayList}; $pa.Add($p)} — EBGreen
– EBGreen, Commented Jun 25, 2018 at 19:43

EBGreen · Accepted Answer · 2018-06-26 00:25:41Z

1

In your code you create one object ($p) then you assign that one object to five different locations in the arraylist. If you want five different objects then you need to create five different objects:

$pa = New-Object System.Collections.ArrayList
for($i=1;$i -le 5;$i++){
    $p = New-Object PSObject -Property @{PIndex = New-Object System.Collections.ArrayList}
    $pa.Add($p)
}

answered Jun 26, 2018 at 0:25

EBGreen

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1 Comment

Sorizon Over a year ago

Thank you.... will go with your suggestion (did have similar code with creating new object instances in a loop elsewhere in my program. Was trying to shortcut it with $pa = @($p) * 5 )... :)

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