Is it possible to create specific constant elements of a PowerShell array?

Question

Let's say I have array Grid that was initialized with

$Grid = @(@(1..3), @(4..6), @(7..9))

and I want to change Grid[0][0] to value "Test" but I want to make it unchangeable, is there a way I could to that?

So far I've tried playing around with classes that allow read-only or constant declarations through the class as opposed to using New-Variable/Set-Variable but it doesn't affect the index itself but the individual element as in

$Grid[0][0] = [Array]::AsReadOnly(@(1,2,3))   
$Grid[0][0]                                   # 1 \n 2 \n 3
$Grid[0][0].IsReadOnly                        # True
$Grid[0][0] = "test"
$Grid[0][0]                                   # test

I assume this is due to $Grid[0][0] being read-only as opposed to constant and the behaviour I experienced supported that:

$test = [Array]::AsReadOnly(@(1,2,3,4))
$test[0]=1                                    # Errors
$test = "test"
$test                                         # test

$Grid = @(@(1..3), @(4..6), @(7..9))
$Grid[0][0] = [Array]::AsReadOnly(@(1,2,3))   
$Grid[0][0][0] = 1                            # Errors
$Grid[0][0] = "test"
$Grid[0][0]                                   # test

I'm not sure what to try next and I know that this is very simple with classes but I am not looking for that as a solution.

Regardless of how you convert the inner array to a read-only collection, the outer array ($grid) is still just a regular array. Given PowerShell's love for enumerating/flattening array expressions, this is going to become near-unreadable to construct as a literal - can you perhaps describe the underlying problem you're trying to solve instead? — Mathias R. Jessen
– Mathias R. Jessen, Commented Apr 9, 2021 at 18:07
@MathiasR.Jessen Sure! Sorry for the ambiguity, the underlying question is how can I define a constant element in a manner like $var = value as opposed to using Set-Variable and nv. I'm not sure how else to describe this in generality but if you have any specific questions I can answer them. — Nico Nekoru
– Nico Nekoru, Commented Apr 9, 2021 at 18:09
You can't, arrays in .NET have no such facility - you'll have to define a custom collection type for this purpose (you might be able to do so with PowerShell classes, but I'd suggest using C# instead) - again, knowing which real-life problem you're trying to solve might be helpful here :) — Mathias R. Jessen
– Mathias R. Jessen, Commented Apr 9, 2021 at 18:11
@MathiasR.Jessen I've taken issue with the hideous array of arrays literal syntax before along with the flattening issue. If memory serves I got around it by using [ArrayList] instead. It didn't seem to have the flattening issue and was syntactically cleaner. Array Lists are otherwise pretty seamless, so it worked out. — Steven
– Steven, Commented Apr 9, 2021 at 20:01

Mathias R. Jessen · Accepted Answer · 2021-04-09 23:59:56Z

You'll have to make both dimensions of your nested array read-only to prevent anyone from overwriting $grid[0]:

$grid = 
  [array]::AsReadOnly(@(
    ,[array]::AsReadOnly(@(1,2,3))
    ,[array]::AsReadOnly(@(3,2,1))
  ))

(the unary , is not a typo, it prevents PowerShell from "flattening" the resulting read-only collection)

Now $grid should behave as you expect:

$grid[0]         # 1,2,3
$grid[0][0]      # 1
$grid[0][0] = 4  # error
$grid[0] = 4     # error

If you want to be able to prevent writing to individual "cells", you'll have to define a custom type:

using namespace System.Collections

class Grid {
    hidden [int[,]] $data
    hidden [bool[,]] $mask

    Grid([int]$width,[int]$height){
        $this.mask = [bool[,]]::new($width, $height)
        $this.data = [int[,]]::new($width, $height)
    }

    [int]
    Get([int]$x,[int]$y)
    {
        if(-not $this.CheckBounds($x,$y)){
            throw [System.ArgumentOutOfRangeException]::new()
        }

        return $this.data[$x,$y]
    }

    Set([int]$x,[int]$y,[int]$value)
    {
        if(-not $this.CheckBounds($x,$y)){
            throw [System.ArgumentOutOfRangeException]::new()
        }

        if(-not $this.mask[$x,$y])
        {
            $this.data[$x,$y] = $value
        }
        else
        {
            throw [System.InvalidOperationException]::new("Cell [$x,$y] is currently frozen")
        }
    }

    Freeze([int]$x,[int]$y)
    {
        if(-not $this.CheckBounds($x,$y)){
            throw [System.ArgumentOutOfRangeException]::new()
        }

        $this.mask[$x,$y] = $true
    }

    Unfreeze([int]$x,$y)
    {
        if(-not $this.CheckBounds($x,$y)){
            throw [System.ArgumentOutOfRangeException]::new()
        }

        $this.mask[$x,$y] = $false
    }

    hidden [bool]
    CheckBounds([int]$x,[int]$y)
    {
        return (
            $x -ge $this.data.GetLowerBound(0) -and 
            $x -le $this.data.GetUpperBound(0) -and 
            $y -ge $this.data.GetLowerBound(1) -and 
            $y -le $this.data.GetUpperBound(1)
        )
    }
}

Now you can do:

$grid = [Grid]::new(5,5)

$grid.Set(0, 0, 1)     # Set cell value
$grid.Get(0, 0)        # Get cell value

$grid.Freeze(0, 0)     # Now freeze cell 0,0
$grid.Set(0, 0, 2)     # ... and this will now throw an exception

$grid.Set(0, 1, 1)     # Setting any other cell still works

If you want native support for index expressions (ie. $grid[0,0]), the Grid class will need to implement System.Collections.IList

Great answer! I probably wouldn't have thought about declaring the outer array. One more question, I have looked through the array members and from this answer I assume that there is no way to declare a constant variable besides New-Variable and Set-Variable? Is that right? Also congrats on 100k it seems like I missed it!

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