Converting an Excel file into CSV file in PowerShell

The issues I encountered with your code are

$outputpath = (Resolve-path $outputpath).Path

Presumably this CSV file doesn't exist, so when you try to resolve the path it gives you nothing.

if(!$outputPath)
{
    $outputPath = $_.DirectoryName
}
$CSVfilename =join-path $outputpath $_.BaseName
$CSVfilename+=".csv";

If you don't pass in an outputpath argument, then this will build the file properly. However, if you pass in an outputpath as a csvfile, then you build out a path that looks like

"N:\Disease_Prevention\...\Output.csv\inputpath basename.csv"

as you're appending the current file's basename and .csv to an already full path.

The adjusted code below worked fine.

param(
    $inputPath = "N:\Disease_Prevention\....\Samplename.xlsx",
    $outputPath = "N:\Disease_Prevention\...\Output.csv"
)

$xlCSV=6
$inputPath = (Resolve-path $inputPath).Path

get-childitem $inputPath -File | foreach {
    write-host "processing $_ " 
    $Excelfilename = $_.fullname
    if(!$outputPath)
    {
        $outputPath = $_.DirectoryName
        $CSVfilename = join-path $outputpath $_.BaseName
        $CSVfilename+=".csv"
    }
    else{
        $CSVfilename = $outputPath
    }

    #open excel and save 
    $Excel = New-Object -comobject Excel.Application
    $Excel.Visible = $False
    $Excel.displayalerts=$False
    $Workbook = $Excel.Workbooks.Open($ExcelFileName)
    $Workbook.SaveAs($CSVfilename,$xlCSV)
    $Excel.Quit()
    If(ps excel){
        kill -name excel
    }

}

I also encourage you to check out Doug Finke's ImportExcel powershell module. Makes working with Excel files so much easier and it doesn't require Excel be installed on the machine that runs the script.