Powershell: You cannot call a null-valued expression - powershell

Question

Hello Stack Overflow Community,
at the moment I'm struggling with this code (it's not that beautiful):
$filepath = "C:\inetpub\logs\LogFiles"
$filearchivepath = "C:\inetpub\logs"
$daystoarchive = 1
$_ = "";
function create-7zip([String] $aDirectory, [String] $aZipfile){
#change the path where you downloaded the 7z exe
[string]$pathToZipExe = "C:\Users\kschweiger\Downloads\7za.exe";
[Array]$arguments = "a", "-tzip", "$aZipfile", "$aDirectory";
& $pathToZipExe $arguments;
}
#Create a new folder with the specific date
$ArchiveFolder = (Get-Date -Format dd.MM.yyyy) + " - Logs-Archive"
if(Test-Path "$filearchivepath\$ArchiveFolder"){
Write-Host "Folder already exists!"
}else{
New-Item -Path $filearchivepath -Name $ArchiveFolder -ItemType directory
}
#Save alle files older than X days into $Files
$Files = Get-ChildItem -Path $filepath -Recurse | where {$_.LastWriteTime -lt (Get-Date).AddDays(-$daystoarchive)}
#Copy/Move files and keep folder structure
foreach ($File in $Files){
$NewPath = $File.DirectoryName.Replace($filepath,"")
if (!(Test-Path "$filearchivepath\$ArchiveFolder\$NewPath"))
{
New-Item -Path "$filearchivepath\$ArchiveFolder\$NewPath" -ItemType Directory
}
$File | Copy-Item -Destination "$filearchivepath\$ArchiveFolder\$NewPath"
}
#Compress folder
if(Test-Path "$filearchivepath\$ArchiveFolder.zip"){
Write-Host "Archive-File already exists!"
}else{
#[IO.Compression.ZipFile]::CreateFromDirectory("$filearchivepath\$ArchiveFolder","$filearchivepath\$ArchiveFolder.zip")
create-7zip "$filearchivepath\$ArchiveFolder" "$filearchivepath\$ArchiveFolder.zip"
#Delete Folder
Remove-Item -Path "$filearchivepath\$ArchiveFolder" -Recurse -Force
}
The code works. but I also get a error message called:
You cannot call a null-valued expression
How can I resolve this?

Answer 1

Get-ChildItem by default returns files and folders. If you need only files, you should use -File. Otherwise, your $Files will contain folders too (as they have LastWriteTime property).
If you try to run .DirectoryName.Replace($filepath,"") on a folder, it'll return such error as you cannot run replacing on $null.
Update: for PowerShell 2.0 you can use | where { ! $_.PSIsContainer } (source)
How can I troubleshoot it by myself?
In your error you can see which line is broken:
$NewPath = $File.DirectoryName.Replace($filepath,"")
All you have to do to troubleshoot such situations is to list all the involved variables and check their values. You could do it like this:
$File
$File.DirectoryName
Pause
$NewPath = $File.DirectoryName.Replace($filepath,"")
Using Pause can be useful as it'll wait for you to press Enter before continuing.