How can I create an immutable array?

Question

I want to create an (immutable) tuple (or array) of elements in Powershell, so my attempt is the following:

$t = @("A","B")

Now, this creates an array that I can add to:

$t += "C"

I want $t to be immutable during the execution of the program. How can I do this?

Seems i misread the explanation i found. Thanks for clearing that up. I deleted my comment so that i don't confuse others. — Snak3d0c
– Snak3d0c, Commented Apr 19, 2018 at 10:24

Bruce Payette · Accepted Answer · 2018-04-19 13:35:30Z

6

You could actually use tuples

PS[1] (203) > $t = [tuple]::create(1,2,3)
PS[1] (204) > $t.item1
1
PS[1] (205) > $t.item1 = 4
'item1' is a ReadOnly property.
At line:1 char:1
+ $t.item1 = 4
+ ~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : PropertyAssignmentException

answered Apr 19, 2018 at 13:35

Bruce Payette

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1 Comment

EBGreen Over a year ago

This is great. I'm sorry to see that tuples are kind of a second class citizen and don't get all the properties and methods that come built in for something like an array but still nice to know.

Janne Tuukkanen · Accepted Answer · 2018-04-19 09:30:25Z

4

You can create Read-Only or Constant variables: (Read-Only variables can be modified using -Force switch with Set-Variable method)

New-Variable -Name foo -Option Constant -Value @("A", "B")
$foo += "C"
Cannot overwrite variable foo because it is read-only or constant.

answered Apr 19, 2018 at 9:30

Janne Tuukkanen

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3 Comments

Palle Due Over a year ago

That's not immutable. This gives no error: $foo[0] = "C"

Sambardo Over a year ago

You'd need to make each value in it a constant as well, but this could work. Using actual tuples as bruce's answer shows is probably better.

Lance U. Matthews Over a year ago

This is akin to applying the readonly modifier to a field in C#: the variable itself is read-only but not necessarily the object it references.

EBGreen · Accepted Answer · 2018-04-19 12:33:44Z

Powershell simply doesn't have the concept of immutability as I understand it. You can fake it with custom objects sort of. Here is some code that can be used to fake an immutable hash table:

function New-ImmutableObject($object) {
    $immutable = New-Object PSObject

    $object.Keys | %{ 
        $value = $object[$_]
        $closure = { $value }.GetNewClosure()
        $immutable | Add-Member -name $_ -memberType ScriptProperty -value $closure
    }

    return $immutable
}
$immutable = New-ImmutableObject @{ Name = "test"}
$immutable.Name = "test1" # Throws error

Not my code. It comes from this quite nice article Functional Programming in PowerShell

You should be able to extend this to whatever type of object you want.

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