A function boilerplate for Excel processing by powershell.

ExcelBoulerplate.ps1

function BoilerplateFunction {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory=$true, ValueFromPipeline=$true)]
        [IO.FileInfo[]]$files
    )
    begin {
        $xls = New-Object -ComObject Excel.Application
        $xls.Visible = $false
        $xls.DisplayAlerts = $false
        $books = $xls.WorkBooks
        
        function Cleanup {
            CleanupObjects @($xls, $books)
        }
    }
    process {
        try {
            foreach ($f in $files) {
                
            }
        }
        catch {
            Cleanup
            throw
        }
    }
    end {
        Cleanup
    }
}

function CleanupObjects($objects) {
    foreach ($obj in $objects) {
        [Runtime.InteropServices.Marshal]::FinalReleaseComObject($obj) | Out-Null
    }
}

function BoilerplateFunctionInternal($books, [IO.FileInfo]$file) {
    $book = $books.Open($file.FullName)
    $sheets = $book.Sheets
    $sheet = $sheets.Item(1)
    $cells = $sheet.Cells
    
    try {
        SomeProcess $cells 
    }
    finally {
        $book.Close();
        CleanupObjects @($cells, $sheet, $sheets)
    }
}

function SomeProcess($cells) {

}