Powershell Writing to .XLSX is Corrupting the Files

Question

I have a Powershell script that loops through .xslx files in a folder and password protects them with the file name (for now.) I have no problem looping through and writing to .xls, but when I try to open an .xlsx file after writing it with Powershell - I get the error:

Excel cannot open the file 'abcd.xlsx' because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

Here's the script:

function Release-Ref ($ref) { 
    ([System.Runtime.InteropServices.Marshal]::ReleaseComObject( 
    [System.__ComObject]$ref) -gt 0) 
    [System.GC]::Collect() 
    [System.GC]::WaitForPendingFinalizers()  
    } 

$e = $ErrorActionPreference
$ErrorActionPreference="continue"
foreach ($f in Get-ChildItem "C:"){
    try{
        $ff = $f
        $xlNormal = -4143 
        $s = [System.IO.Path]::GetFileNameWithoutExtension($f)
        $xl = new-object -comobject excel.application 
        $xl.Visible = $False
        $xl.DisplayAlerts = $False   
        $wb = $xl.Workbooks.Open($ff.FullName)
        $wb.sheets(1).columns("A:S").entirecolumn.AutoFit()
        $wb.sheets(1).columns("N").NumberFormat = "0.0%"
        $a = $wb.SaveAs("C:\Out\" + $s + ".xls",$xlNormal,$s) #works
        #$a = $wb.SaveAs("C:\Out\" + $s + ".xlsx",$xlNormal,$s) #doesn't work
        $a = $xl.Quit() 

        $a = Release-Ref($ws) 
        $a = Release-Ref($wb) 
        $a = Release-Ref($xl) 
    }
    catch {
        Write-Output "Exception"
        $ErrorActionPreference=$e;
    }
}

I've searched other questions but can't find any other examples of the same issues writing from Powershell. Thank you.

Answer 1

The problem is caused because Xls is different a format from Xlsx. Older Excels before version 2007 used binary formats. The 2007 Office introduced new formats called Office Open Xml, which Xslx uses.

Excel is smart enough to check both file extension and file format. Since saving a binary file with new versions' extension creates a conflict, the error message hints for this possibility too:

and that the file extension matches the format of the file.

Why doesn't Excel just open the file anyway? I guess it's a security feature that prevents unintentional opening of Office documents. Back in the days, Office macro viruses were a bane of many offices. One of the main infection vectors was to trick users to open files without precautions. Unlike classic viruses, macro ones infected application data (including default template files) instead of OS binaries, but that's another a story.

Anyway, to work in proper a format, use proper version value. That would be -4143 for Xls and 51 for Xlsx. What's more, Get-ChildItem returns a collection of FileInfo objects, and file extension is there in Extension property. Like so,

# Define Xls and Xlsx versions
$typeXls = -4143
$typeXls = 51
foreach ($f in Get-ChildItem "C:"){

    try{
    $ff = $f
    ...
   # Select saveas type to match original file extension
   if($f.extension -eq '.xsl') { $fType = $typeXls }
   else if($f.extension -eq '.xslx') { $fType = $typeXlsx }

   $a = $wb.SaveAs("C:\Out\" + $s + $.extension, $fType, $s)