Powershell if file is different then do something - powershell

Question

As the title suggests, how do I do an if statement for comparing 2 files? This is what I have so far but even though the files are the same it still keeps prompting me for an update
Add-Type -AssemblyName 'PresentationFramework'
$continue = 'Yes'
Invoke-WebRequest -Uri "https://<url>/file" -OutFile "C:\temp\file"
if ( "C:\temp\file" -ne "C:\app\file" ) {
$caption = "Update Available!"
$message = "There is a new version available. Do you want to update to the latest version?"
$continue = [System.Windows.MessageBox]::Show($message, $caption, 'YesNo');
} else {
Write-Output "You have the latest version!"
}
if ($continue -eq 'Yes') {
Write-Output "Downloading application Please wait..."
// Do something //
} else {
Write-Output "Cancelled"
}
Any help would be greatly appreciated! Thanks

Answer 1

This is the logic I would personally follow, you can re-estructure it later for your need but in very basic steps:
Get the MD5 sum of my current file.
Query the new file with Invoke-WebRequest and using a MemoryStream check if the new hash is the same as the previous one.
If it's not, write that stream to a FileStream.
$ErrorActionPreference = 'Stop'
try {
$previousFile = "C:\app\file"
$previousHash = (Get-FileHash $previousFile -Algorithm MD5).Hash
$webReq = Invoke-WebRequest 'https://<url>/file'
$memStream = [IO.MemoryStream] $webReq.Content
$newHash = (Get-FileHash -InputStream $memStream -Algorithm MD5).Hash
if($previousHash -ne $newHash) {
$file = [IO.File]::Create('path\to\mynewfile.ext')
$memStream.WriteTo($file)
}
}
finally {
$file, $memStream | ForEach-Object Dispose
}
The main idea behind it is to check all in memory so as to avoid the need to download a file if not needed.