null-valued expression error in simple powershell script - powershell

Question

I have a small powershell script that looks in a patch for all files that contain a string and then replace that string.
Im sure this worked last week so im very confused why its now not working.
$filePath = "C:\my\file\path*"
# Get the files from the folder and iterate using Foreach
Get-ChildItem $filePath -Recurse | ForEach-Object {
# Read the file and use replace()
(Get-Content $_).Replace('oldString','NewString') | Set-Content $_
}
Im getting two errors i think, the first is:
Get-Content : Access to the path 'C:\my\file\path\YYY' is denied.
YYY is a folder in my path and im running the script as administrator, i was running as my own user who i confirmed has full access to this path.
The second is:
You cannot call a method on a null-valued expression.
Im guessing its the $_ but im really not sure. Ive tried replacing it with different name but no luck.

Answer 1

The You cannot call a method on a null-valued expression error is coming from calling the replace method when the result of Get-Content $_ is null.
This can happen for a few reasons:
First, inside of your ForEach-Object script block, $_ will essentially return the same value as $_.name which does not contain the full path to the folder or file being processed. Depending on the value of the working directory,$PWD, when you execute this code this could result in a bad file or folder path.
To address issues with the working directory try using $_.FullName. For debugging you can also write that value to the terminal to confirm where it is looking.
The issue could also be that the file has no contents. If the file is empty then the result of Get-Content will be $null.
Also attempting to get the content of a folder will result in an "access denied" error, after which the result of (get-content $_) will also be $null, resulting in the null error you received.
To summarize:
Double check your working directory.
Consider using $_.FullName.
Consider adding code to avoid calling Get-Content on folders.
Here is an example which I tested with several nested folders, some of which contained empty files and others which contained files with contents:
Get-ChildItem C:\testFolder -File -Recurse | ForEach-Object{
#this write host is just for debugging, to see what the file path is
Write-Host $_.FullName
$fileContents = Get-Content $_.FullName
if($fileContents -ne $null){
$fileContents.replace('oldString','newString') | Set-Content $_.FullName
}
}