PowerShell add a function to a custom object

Question

I try to add an existing function as a method to a new created object. Writing an inline function works:

$myObject | Add-Member ScriptMethod -name Calc -value{param([int]$a,[int]$b;$a+$b}

Having a function:

function get-Calc{param([int]$a,[int]$b) $a +$b}

this doesn't work:

$myObject | Add-Member ScriptMethod -name Calc -value(get-Calc)

What's the point of adding it as a method on a specific object if the function doesn't use the object in any way? — Mathias R. Jessen
– Mathias R. Jessen, Commented Feb 7, 2022 at 14:58
but I like to use this method, like that: $myObject.Calc(2,3) — Purclot
– Purclot, Commented Feb 7, 2022 at 15:01
I get that, I just don't understand why :) Normally you'd attach code that references the object itself, eg.: $myObject = [pscustomobject]@{ MyValue = 123 } |Add-Member -Name Times -Value {param([int]$X) return $this.MyValue * $X} -PassThru -MemberType ScriptMethod, which would then make $myObject.Times(2) evaluate to 246 — Mathias R. Jessen
– Mathias R. Jessen, Commented Feb 7, 2022 at 15:03

Santiago Squarzon · Accepted Answer · 2022-12-06 14:58:39Z

5

You could also do it passing a script block as -Value, see the Constructor for PSScriptMethod Class.

function Get-Calc { param([int] $a, [int] $b) $a + $b }

$myObject = [pscustomobject]@{
    A = 1
    B = 2
}

# If you're referencing the existing Properties of the Object:
$myObject | Add-Member ScriptMethod -Name MyMethod1 -Value {
    Get-Calc $this.A $this.B
}

# If you want the Instance Method to reference the arguments being passed:
$myObject | Add-Member ScriptMethod -Name MyMethod2 -Value {
    param([int] $a, [int] $b)

    Get-Calc $a $b
}

# This is another alternative as the one above:
$myObject.PSObject.Methods.Add(
    [psscriptmethod]::new(
        'MyMethod3', {
            param([int] $a, [int] $b)

            Get-Calc $a $b
        }
    )
)

$myObject.MyMethod1()
$myObject.MyMethod2(1, 2)
$myObject.MyMethod3(3, 4)

Note: All examples in this answer require that the function Get-Calc is defined in the parent scope and will fail as soon as the definition for the function has changed. Instead you should pass in the definition as a Script Block as Mathias's helpful answer is showing.

edited Dec 6, 2022 at 14:58

answered Feb 7, 2022 at 15:10

Santiago Squarzon

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1 Comment

Santiago Squarzon Over a year ago

@zett42 It was meant to show the alternative to Add-Member

Mathias R. Jessen · Accepted Answer · 2022-02-07 15:04:08Z

3

Defined functions are stored in the special function:\ drive.

To reference a function definition using variable syntax:

$myObject | Add-Member ScriptMethod -name Calc -Value ${function:get-Calc}

answered Feb 7, 2022 at 15:04

Mathias R. Jessen

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Comments

Sage Pourpre · Accepted Answer · 2022-02-07 15:11:07Z

1

There are 2 problems with your example that do not work.

You are not passing Get-Calc as a scriptblock (The expected value for a scriptmethod is a scriptblock)
You are not passing your expected parameters

Instead of

$myObject | Add-Member ScriptMethod -name Calc -value(get-Calc)

Do this:

$myObject | Add-Member ScriptMethod -name Calc3 -value {param([int]$a,[int]$b) get-Calc -a $a -b $b}

answered Feb 7, 2022 at 15:11

Sage Pourpre

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