How to dynamically add PSCustomObjects to a list [duplicate] - powershell

Question

This question already has answers here:
Add items into a collection array dynamically
(2 answers)
Closed 1 year ago.
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)
}