How to dynamically add PSCustomObjects to a list

Question

I am creating a script to parse a CSV file, where I store the content of each indexed field in the CSV as a NoteProperty in a PSCustomObject.

As I parse the file line by line, I add the PSCustomObject to a list type. When I output my list, I want to be able to do something like:

 $list | Format-Table

and have a nice view of each row in the csv file, separated into columns with the heading up top.

Problem

When I add a PSCustomObject to the list, it changes the type of the list to a PSCustomObject. In practice, this has the apparent effect of applying any updates made to that PSCustomObject to every element in the list retroactively.

Here is a sample:

 $list  = [System.Collections.Generic.List[object]]::new()
 $PSCustomObject    = [PSCustomObject]@{ count  = 0}
 Foreach ($i in 1..5) {
    $PSCustomObject.count +=1
    $list.Add($PSCustomObject)
 }

Expected Output:

PS>$list
    count
    -----
        1
        2
        3
        4
        5

Actual Output:

PS>$list
    count
    -----
        5
        5
        5
        5
        5

Question

Is there any way to get the expected output?

Limitations / additional context if it helps

I'm trying to optimize performance, as I may parse very large CSV files. This is why I am stuck with a list. I understand the Add method in lists is faster than recreating an array with += for every row. I am also using a runspace pool to parse each field separately and update the object via $list.$field[$lineNumber] = <field value>, so this is why I need a way to dynamically update the PSCustomObject. A larger view of my code is:

    $out = [hashtable]::Synchronized(@{})
    $out.Add($key, @{'dataSets' = [List[object]]::new() } )    ### $key is the file name as I loop through each csv in a directory.
    $rowTemplate = [PSCustomObject]@{rowNum = 0}

    ### Additional steps to prepare the $out dictionary and some other variables
    ...
    ...
    try {
        ### Skip lines prior to the line with the headers
        $fileParser = [System.IO.StreamReader]$path
        Foreach ( $i in 1..$headerLineNumber ) {
            [void]$fileParser.ReadLine()
        }
        ### Load the file into a variable, and add empty PSCustomObjects for each line as a placeholder.
        while ($null -ne ($line = $fileParser.ReadLine())) { 
            [void]$fileContents.Add($line)
            $rowTemplate.RowNum += 1
            [void]$out.$key.dataSets.Add($rowTemplate)
        }
    }
    finally {$fileParser.close(); $fileParser.dispose()}
    ### Prepare the script block for each runspace
    $runspaceScript = {
        Param( $fileContents, $column, $columnIndex, $delimiter, $key, $out )
        $columnValues   = [System.Collections.ArrayList]::new()
        $linecount      = 0

        Foreach ( $line in $fileContents) {

            $entry = $line.split($delimiter)[$columnIndex]
            $out.$key.dataSets[$linecount].$column = $entry
            $linecount += 1
        }
    }
    ### Instantiate the runspace pool.

PS Version (5.1.19041)

Answer 1

You're (re-)adding the same object to the list, over and over.

You need to create a new object every time your loop runs, but you can still "template" the objects - just use a hashtable/dictionary instead of a custom object:

# this hashtable will be our object "template"
$scaffold = @{ Count = 0}

foreach($i in 1..5){
  $scaffold.Count += 1
  $newObject = [pscustomobject]$scaffold

  $list.Add($newObject)
}

---

As mklement0 suggests, if you're templating objects with multiple properties you might want to consider using an ordered dictionary to retain the order of the properties:

# this hashtable will be our object "template"
$scaffold = [ordered]@{ ID = 0; Count = 0}

foreach($i in 1..5){
  $scaffold['ID'] = Get-Random
  $scaffold['Count'] = $i
  $newObject = [pscustomobject]$scaffold

  $list.Add($newObject)
}