Powershell filter for only unique entries in array from pipeline within custom module

Question

This is a tangent/continuation from my other question where I need to sanitize the input to one of my custom modules. Here's the code:

function Get-MyTest {

    param(
        [string[]]$MyInput
    )


    [array]$Output = @()
    $i=0

    foreach($Item in $MyInput){

        $i++

        $Output += [pscustomobject]@{
            Name = $Item
            Id = $i
        }

    }

    return $Output

}

function Get-IncorrectResults {

    param(
        [parameter(ValueFromPipeline)][array]$MyIncorrectInput
    )

    process {
    
        foreach ($Item in $MyIncorrectInput) {
        
            Write-Host "Current object: $Item `n "

        }

    }

}

Problem
Adding begin{}, process{}, and end{} allows me to individually process each entry in the pipeline, but I can no longer filter the input. A user could end up with several identical objects in the $MyIncorrectInput, for example by doing:

[array]$TestVariable = @()
$TestVariable += Get-MyTest "Test1","Test2"
$TestVariable += Get-MyTest "Test3","Test2"

This would end up with two identical entries for Test2. If I then run either of the following:

Get-IncorrectResults -MyIncorrectInput $TestVariable
$TestVariable | Get-IncorrectResults

Then the Test2 object will be processed twice. Since the goal here is to use this in GET/POST/PUT/DELETE requests for a RESTAPI then I can't have the code process the same entry (or ID) twice for DELETE since the resource has already been deleted. I therefore need to filter out to only process unique values.

Attempted solution
I tried to change the foreach loop to use $item in $UniqueInput instead, and add the following code:

$UniqueInput = $MyIncorrectInput | Get-Unique -AsString

If I add it in the process{} block it doesn't do anything, it still processes every entry including duplicates. If I add it in the begin{} block well then the value doesn't seem to carry over to the process{} block and the Write-Host section is empty because $UniqueInput seems to be empty.

Even changing to the following has no effect:

$UniqueInput = $MyIncorrectInput | Select-Object * -Unique

What is the correct way to filter an array input for only unique values in a custom module that needs to take the array from the pipeline? I don't need to have the full PSCustomObject be unique, it would be enough to have the .Id unique if that makes any difference.

I'm guessing you're looking for $TestVariable | Sort-Object { $_.Id } -Unique | Get-IncorrectResults but not totally sure — Santiago Squarzon
– Santiago Squarzon, Commented Sep 8, 2022 at 2:51
Oh, yeah, that's right, I was going about it the wrong way, of course! Filter values in process, then run the foreach in the end block. Amazingly quick response, thank you so much! If you want to post it as an answer I'll accept it :) — Tanaka Saito
– Tanaka Saito, Commented Sep 8, 2022 at 2:53
So what worked was put [array]$UniqueInput = @() in the begin block, put $UniqueInput += $MyIncorrectInput in the process block, and then put $UniqueInput = $UniqueInput | Sort-Object Id -Unique in the beginning of the end block and then run my RESTAPI calls. Thank you guys so much! — Tanaka Saito
– Tanaka Saito, Commented Sep 8, 2022 at 2:59

Santiago Squarzon · Accepted Answer · 2022-09-09 12:31:39Z

You can do this with a HashSet<T>, this class is designed to contain only unique elements:

function Get-IncorrectResults {
    param(
        [parameter(Mandatory, ValueFromPipeline)]
        [array] $MyIncorrectInput,

        [parameter(Mandatory)]
        [string] $On,

        [parameter()]
        [switch] $CaseSensitive
    )

    begin {
        if($CaseSensitive.IsPresent) {
            return $hash = [Collections.Generic.HashSet[string]]::new()
        }
        $hash = [Collections.Generic.HashSet[string]]::new([StringComparer]::InvariantCultureIgnoreCase)
    }
    process {
        foreach ($Item in $MyIncorrectInput) {
            if($hash.Add($Item.$On)) {
                $Item
            }
        }
    }
}

$TestVariable = @(
    Get-MyTest "Test1", "Test2"
    Get-MyTest "Test3", "Test2"
)

Get-IncorrectResults $TestVariable -On Id
$TestVariable | Get-IncorrectResults -On Id

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