PowerShell scope clash when dot sourcing scripts

Question

I'm having some scope issues when dot sourcing powershell scripts. Suppose I have one script 'A.ps1':

$VERSION = "1.0"

# Dot source B.ps1
. .\B.ps1

function Write-Version { Write-Host "A.ps1 version $VERSION" }
Write-Version

And a script B.ps1

$VERSION = "2.0"
function Write-Version { Write-Host "B.ps1 version $VERSION" }
Write-Version

The output of running A.ps1 will be:

B.ps1 version 2.0
A.ps1 version 2.0

Why this happens is quite obvious. The $VERSION variable from B.ps1 is put into the scope of A.ps1 and overwrites that variable. Indeed, this happens with Write-Version as well, but here A.ps1 overwrites B's version, but because Write-Version is called in B.ps1 before that happens, we can still see the output of B's Write-Version function.

The question, of course, is how to prevent this?? I've tried various scope options, but this doesn't seem to work when dot-sourcing. And since there are functions in B.ps1 that I do need in A's scope, just invoking B.ps1 is probably not an option.

Does anyone have any ideas?

Answer 1

You can do it by making B.ps1 a module and renaming it to B.psm1. Add Export-ModuleMember to make your functions available to other scripts.

This would be B.psm1:

$VERSION = "2.0"
function Write-Version { Write-Host "B.ps1 version $VERSION" }
Write-Version

# Only items specified here will be exported. If Export-ModuleMember is not used,
# ALL members (functions, variables, and aliases) will be exported. In this case
# if $VERSION was exported, $VERSION will be set to "2.0" in script A.ps1
Export-ModuleMember -Function Write-Version

And A.ps1 would be:

$VERSION = "1.0"

# Import B.psm1
Import-Module .\B.psm1

function Write-Version { Write-Host "A.ps1 version $VERSION" }
Write-Version

# Use B.psm1's `Write-Version` function
B\Write-Version