Create an array off objects as a class member in PowerShell

Question

I keep getting errors when running the code below. I don't understand what I'm doing wrong

ERROR 1: You cannot call a method on a null-valued expression. At line:25 char:9

ERROR 2: Cannot index into a null array. At line:43 char:1

CODE:

            Class Step {
                [int]$StepNumber = 0
                [string]$Name = ''
                [string]$ScriptFile = ''
                [int]$status = 0 # 0 = 'InComplete', 1 = 'Complete', 2 = 'Failed', -1 = 'Skipped'
                [string]$DependencyStepNumber = -1

                Step([string]$Name, [int]$StepNumber, [string]$ScriptFile) {
                    $this.Name = $Name
                    $this.StepNumber = $StepNumber
                    $this.ScriptFile = $ScriptFile
                }
            }


            Class Guide {
                [int]$StepNumberCounter = 0
                [string]$Name = ''
                [Step[]]$Steps

                [int]AddStep([string]$Name, [string]$ScriptFile) {
                    $newStepNumber = $this.GetNewStepNumber()
                    $newStep = [Step]::new($Name, $newStepNumber, $ScriptFile)

line-25>>>          $this.Steps.Add($newStep)

                    return $newStepNumber
                }

                [int]GetStepName([int]$StepNumber) {
                    return $this.Steps[$StepNumber-1]
                }

                [int]GetNewStepNumber() {
                    return $this.StepNumberCounter += 1
                }
             }

             $Guide = [Guide]::new()
             $Guide.AddStep('Step One','ScriptOne.ps1')
             $Guide.AddStep('Step Two','ScriptTwo.ps1')

line-43>>>   Write-Host $Guide.Steps[0]
             Write-Host $Guide.Steps[1]

Please indicate which is line 25 and 43.

EBGreen
– EBGreen

2018-08-01 16:18:43 +00:00
Commented Aug 1, 2018 at 16:18 — EBGreen
– EBGreen, Commented Aug 1, 2018 at 16:18

mayursharma · Accepted Answer · 2018-08-01 18:20:17Z

2

Array list has better performance, see below revised code

Class Step {
    [int]$StepNumber = 0
    [string]$Name = ''
    [string]$ScriptFile = ''
    [int]$status = 0 # 0 = 'InComplete', 1 = 'Complete', 2 = 'Failed', -1 = 'Skipped'
    [string]$DependencyStepNumber = -1

    Step([string]$Name, [int]$StepNumber, [string]$ScriptFile) {
        $this.Name = $Name
        $this.StepNumber = $StepNumber
        $this.ScriptFile = $ScriptFile
    }
}


Class Guide {
    [int]$StepNumberCounter = 0
    [string]$Name = ''
    [System.Collections.ArrayList]$Steps = @()

    [int]AddStep([string]$Name, [string]$ScriptFile) {
        $newStepNumber = $this.GetNewStepNumber()
        $newStep = [Step]::new($Name, $newStepNumber, $ScriptFile)

        $this.Steps.Add($newStep)

        return $newStepNumber
    }

    [int]GetStepName([int]$StepNumber) {
        return $this.Steps[$StepNumber-1]
    }

    [int]GetNewStepNumber() {
        return $this.StepNumberCounter += 1
    }
}

$Guide = [Guide]::new()

$Guide.AddStep('Step One','ScriptOne.ps1')
$Guide.AddStep('Step Two','ScriptTwo.ps1')

Write-Output $Guide.Steps[0]
Write-Output $Guide.Steps[1]

answered Aug 1, 2018 at 18:20

mayursharma

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1 Comment

jarobinson Over a year ago

Yes, this solves it with my original intent of using an ArrayList, thanks.

Jean-Claude Colette · Accepted Answer · 2018-08-01 16:54:50Z

2

To add an element to an array, we use the += operator.

$this.Steps += $newStep

Add is reserved for ArrayList:

$al = New-Object System.Collections.ArrayList
$al.Add('Example')

answered Aug 1, 2018 at 16:54

Jean-Claude Colette

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2 Comments

HAL9256 Over a year ago

Jean-Claude Colette is correct, switch to using += instead of Add().

jarobinson Over a year ago

Thanks! That was it.

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