Advanced PowerShell hashing functions

You're probably going to want to use ForEach-Object to do multiple things with each file as you cycle through them. The automatic variable $_ will let you target each file within the code block.

Get-ChildItem -Path C:\Windows -Force -Recurse -Include "*.exe","*.dll","*.sqr","*.sqc","*.sql","*.dms","*.asps" -ErrorAction SilentlyContinue | ForEach-Object {
    Get-Hash $_.FullName | Out-File 'results.txt'
    }

I'd think you would want to compare the old hash to the current one, in which case you could perform step 2 while you're going through step 1. If not you could pull the two result sets into variables and compare individual lines with matching filenames.

If you are on PowerShell V4, you can use the -PipelineVariable parameter to stash the current FileInfo object for use further down the pipeline when $_ has been redefined to a different object:

$ext = "*.exe","*.dll","*.sqr","*.sqc","*.sql","*.dms","*.asps"
Get-ChildItem C:\Windows -Force -Recurse -Include $ext -pv fileInfo -ErrorAction SilentlyContinue | 
    Get-Hash | Foreach {$_.HashString + " " + $fileInfo.Name} | 
    Out-File results.txt

If you're not on V4 you can use a foreach-object cmdlet like so:

Get-ChildItem C:\Windows -Force -Recurse -Include $ext -ErrorAction SilentlyContinue | 
    Foreach {$fileInfo = $_; $hash = $_ | Get-Hash; $hash.HashString + " " + $fileInfo.Name} |
    Out-File results.txt

Here is another solution. I would add the hash to the fileobject in a loop, and then I would export in an XML object file. For me it's then more simple to compare objects using Import-Clixml (be careful to use a select to export only the properties you need because Export-Clixml consumes time)

$ext = "*.exe","*.dll","*.sqr","*.sqc","*.sql","*.dms","*.asps"
Get-ChildItem -Path C:\temp\Arielle -Force -Recurse -Include $ext -ErrorAction SilentlyContinue | % {$a=Get-FileHash $_; Add-Member -InputObject $_ -MemberType Noteproperty -name "Hash" -Value $a.Hash; $_} | Export-Clixml 'c:\temp\results.xml'

It looks like you are using the Get-Hash cmdlet from PSCX... You can make your own script cmdlet that outputs all the original object information with the hash (example below) -

For the comparison you can create text files with the file path and hash on the same line -

dir C:\ | Get-FileMd5 | % {$_.FullName + '=' + $_.HashMD5} | Set-Content results.txt

Here's the function -

function Get-FileMd5 {
    begin {
        Add-Type -AssemblyName System.Security
    }
    process {
        $input | % {
            try {
                $path = ($_ | Resolve-Path -ErrorAction SilentlyContinue).Path
                if ($path) {
                    $file = Get-Item -Path $path
                    if ($file -is [System.IO.FileInfo]) {
                        $stream = $file.Open([System.IO.FileMode]::Open)
                        $crypt_prov = New-Object -TypeName System.Security.Cryptography.MD5CryptoServiceProvider
                        $md5_hash = [System.BitConverter]::ToString($crypt_prov.ComputeHash($stream)).Replace('-', '')
                        $stream.Close()
                        $file | Add-Member -MemberType NoteProperty -Name HashMD5 -Value $md5_hash -PassThru
                    }
                }
            } catch {
                if ($stream) {$stream.Close()}
                Write-Error -ErrorRecord $_
            }
        }
    }
}

dir C:\ | Get-FileMd5 | Select *