How to get the content of a file during NuGet package install - nuget-package

Question

I have a nuget package solution which installs just fine. I now need to modify the target project's Properties\AssemblyInfo.cs file to add some code.
I have an Install.ps1 script so I'm adding my powershell script to this. As I'm building it up as I go, what it does at the moment is this:
param($installPath, $toolsPath, $package, $project)
$content = Get-Content $project.ProjectItems.Item("Properties\AssemblyInfo.cs")
The error it is giving me is this:
Value does not fall within the expected range.At
C:\git\Testing\packages\Standards.Testing.1.0.6694.30974-beta\tools\Install.ps1:2
char:1
+ $content = Get-Content $project.ProjectItems.Item("Properties\Assembl ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OperationStopped: (:) [], ArgumentException
+ FullyQualifiedErrorId : System.ArgumentException
The intent is to load the content of the AssemblyInfo.cs file and check it for certain content, then modify it and write it back.
What I don't understand is why it won't read the content of that file into a variable.

Answer 1

The problem was that I was incorrectly trying to reference the path of the AssemblyInfo.cs file. I didn't realise that the parameters to the script were providing everything I need and that other posts have referenced a scary-looking class which contains all the required information.
SO containing information about script parameters:
Need PowerShell Script in NuGet to install selected DLLs from Package into a VS Project
Linked from that post is this page which details the available information about a nuget install:
https://learn.microsoft.com/en-us/dotnet/api/envdte.dte?redirectedfrom=MSDN&view=visualstudiosdk-2017
My script now looks like this:
param($installPath, $toolsPath, $package, $project)
#Update the AssemblyInfo.cs if it has not been updated before
function Get-Append-String {
$text = ''
$args[0] | ForEach-Object -Process {
$text += $_ + "`n"
}
return $text
}
function Get-Append {
$text = ''
$args | ForEach-Object -Process {
$text += $_
}
return $text
}
function Get-Contains {
$text = Get-Append-String $args[0]
return ($text -like $args[1])
}
$query = "*using Xunit;*"
$xunit = "using Xunit;`n"
$comment = "`n// xUnit configuraiton...`n// MaxParallelThreads limits the number of threads which xUnit will use to run tests`n[assembly: CollectionBehavior(MaxParallelThreads = 8)]`n"
$path = $project.FullName + '\..\Properties\AssemblyInfo.cs'
$content = Get-Content -Path $path
$content = Get-Append-String $content
if ( ($content -like $query) -eq $false ) {
$content = $xunit + $content + $comment
Set-Content -Path $path -Value $content
}