Use PowerShell and Excel to convert a set of files from .xlsx to .csv automatically

Convert-ExcelToCsv.ps1

[CmdletBinding()]
param (
    [Parameter(Mandatory=$true)]
    [ValidateScript({ 
        if (Test-Path -Path (Resolve-Path $_) -PathType Container) { return $true } 
        else { throw "The SourcePath is not valid" }})]
    [string]
    $SourcePath,

    [Parameter(Mandatory=$true)]
    [ValidateScript({ 
        if (Test-Path -Path (Resolve-Path $_) -PathType Container) { return $true } 
        else { throw "The DestinationPath is not valid" }})]
    [string]
    $DestinationPath,

    [Parameter()]
    [string]
    $Filter = "*.xlsx"
)

begin {
    # use this in the end block to ensure and Excel processes
    # created by this script are closed, but others are left open

    $existingExcelProcesses = Get-Process *excel* | ForEach-Object { $_.Id }
    $excel = New-Object -COMObject Excel.Application;
}

process {
    $SourcePath = Resolve-Path -Path $SourcePath
    $DestinationPath = Resolve-Path -Path $DestinationPath

    $csvFileName = @{l="CsvName";e={[System.IO.Path]::GetFileNameWithoutExtension($_.Name) + ".csv"}}
    $csvFileType = 6

    Get-ChildItem -Path $SourcePath -Filter $Filter | Select-Object Name,$csvFileName | ForEach-Object {
        $excelPath = Join-Path -Path $SourcePath -ChildPath $_.Name -Resolve
        $csvPath = Join-Path -Path $DestinationPath -ChildPath $_.CsvName
        try {
            $wb = $excel.Workbooks.Open($excelPath)
            $wb.SaveAs($csvPath, $csvFileType)
            $wb.Close(1) # close the document, forcing it to not ask to save
            @{Name = $csvPath;Success=$true}
        } catch {
            Write-Error $_.Exception.Message
            @{Name = $excelPath;Success=$false}
        } 
    } | Select-Object Name,Success
}

end {
    $excel.Quit()

    # for some reason there was always an Excel Process hanging around 
    # that I thought $excel.Quit() would get rid of. I'm probably missing something...
    Get-Process *excel* | Where-Object { -not ($existingExcelProcesses -contains $($_.Id) ) } | Stop-Process
}