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

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.

Answer 1

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