Need Accurate result Size on Disk using Powershell Script, When Bytes are Null

I don't know what files you have in your App-V folder, so I created a few files in a test folder to examine.

I found 2 issues. The first is that really small files don't occupy any space. Their data is stored in the MFT. Explorer properties show 0 bytes for size on disk.

The second issue is for files that have been compressed. You can set the explorer options to have them show in blue.

I found a StackOverflow article that demonstrated how to call GetCompressedFileSizeW. That works for compressed files, but the only way that I found to find the MFT resident files was to call fsutil and look for the number of extents.

This was tested on Win11.

# Source: https://stackoverflow.com/questions/22507523/getting-size-on-disk-for-small-files-in-powershell
add-type -type  @'
using System;
using System.Runtime.InteropServices;
using System.ComponentModel;
using System.IO;
namespace Win32Functions
{
  public class ExtendedFileInfo
  {    
    public static long GetFileSizeOnDisk(string file)
    {
        FileInfo info = new FileInfo(file);
        uint dummy, sectorsPerCluster, bytesPerSector;
        int result = GetDiskFreeSpaceW(info.Directory.Root.FullName, out sectorsPerCluster, out bytesPerSector, out dummy, out dummy);
        if (result == 0) throw new Win32Exception();
        uint clusterSize = sectorsPerCluster * bytesPerSector;
        uint hosize;
        uint losize = GetCompressedFileSizeW(file, out hosize);
        long size;
        size = (long)hosize << 32 | losize;
        return ((size + clusterSize - 1) / clusterSize) * clusterSize;
    }
    [DllImport("kernel32.dll")]
    static extern uint GetCompressedFileSizeW([In, MarshalAs(UnmanagedType.LPWStr)] string lpFileName,
       [Out, MarshalAs(UnmanagedType.U4)] out uint lpFileSizeHigh);
    [DllImport("kernel32.dll", SetLastError = true, PreserveSig = true)]
    static extern int GetDiskFreeSpaceW([In, MarshalAs(UnmanagedType.LPWStr)] string lpRootPathName,
       out uint lpSectorsPerCluster, out uint lpBytesPerSector, out uint lpNumberOfFreeClusters,
       out uint lpTotalNumberOfClusters);  
  }
}
'@
function Get-FolderSizEOnDisk {
param (
[Parameter(Mandatory = $true)]
[string]$path
)
    $ap = (Resolve-Path $path).Path
    $ite = Get-Item $ap
    $volume = Get-Volume $ite.PSDrive.Name
    $cSize = $volume.AllocationUnitSize
    $defaultCS = 4096
    $totalSizeOnDisk = 0
    $filecount =0
    $defaultcount = 0
    $failedcount = 0
    Get-ChildItem -Path $path -Recurse -File -ErrorAction SilentlyContinue | ForEach-Object {
    
    try {
        $size = $_.Length
        if($null -ne $size -and $size -gt 0)
        {
            # $cl = [math]::Ceiling($size / $cSize)
            # $SizeOnDisk = $cl * $cSize
            $SizeOnDisk = [Win32Functions.ExtendedFileInfo]::GetFileSizeOnDisk( $_.FullName)
 
            if ($size -lt 1024) {                    # really small files can be stored in the MFT 
                $fsu = fsutil file queryExtents "$($_.FullName)"
                if ($fsu.contains("No extents")) {
                    $SizeOnDisk = 0                  # data stored in MFT 
                }
            }  
            $totalSizeOnDisk += $SizeOnDisk
            $filecount++
            "{0} - {1} - {2}" -f $size, $SizeOnDisk, $_.Name
        }
    else {
        $totalSizeOnDisk += $defaultCS
        $defaultcount++
        }
    } catch {
        $failedcount++
    }
}
$folderCount = (Get-ChildItem -Path $path -Recurse -Directory -ErrorAction SilentlyContinue).Count
[PSCustomObject]@{
    Folder =$ap
    Files = $filecount
    defaultLength_Zero = $defaultcount
    FailedFiles = $failedcount
    Folders = $folderCount
    SizeOnDisk = $totalSizeOnDisk
    SizeOnDiskKB =[math]::Round($totalSizeOnDisk / 1KB, 2)
    SizeOnDiskGB =[math]::Round($totalSizeOnDisk / 1GB, 2)
    SizeOnDiskMB =[math]::Round($totalSizeOnDisk / 1MB, 2)
    }
}
Get-FolderSizEOnDisk "C:\Temp\store"