Performance#

Practice: Use List[T] instead of @() in loops. Use -Filter over Where-Object. Use .Where() and .ForEach() for large collections.

Why: $results += $item copies the entire array on every iteration — O(n²). List[T] appends in O(1). Provider -Filter is evaluated at the source before data crosses the pipeline. Use the construct that scales (Build for the modern engineer).

How:

# Good
$results = [System.Collections.Generic.List[PSObject]]::new()
foreach ($item in $collection) {
    $results.Add($processedItem)
}

Get-ChildItem -Path C:\Temp -Filter *.txt
$filtered = $collection.Where({ $_.Value -gt 10 })

# Bad
$results = @()
foreach ($item in $collection) {
    $results += $processedItem           # O(n²)
}

Get-ChildItem -Path C:\Temp | Where-Object { $_.Name -like '*.txt' }