Why is $DebugPreference getting lost in a module?

Question

I have a powershell script and I have set the $DebugPreference to "Continue". However when I call Write-Debug from a module that is called from my script, the $DebugPreference changed to "SilentlyContinue". Why is that? How can I keep $DebugPreference the same as the calling script? Example below

CallingScript.ps1

$DebugPreference = "Continue"
Write-Host "Debug preference: $DebugPreference"
Write-Debug "Checking that debugging works"
Import-Module Logging;
Write-Log "Debug" "Checking that debugging still works!"

Logging.psm1

Function Write-Log
{
    param (
    [ValidateSet("Error","Warning","Debug","Info")][String]$type,
    [String]$logMessage
    )
    Write-Host "Debug preference: $DebugPreference"
    switch($type)
    {
        "Error" {Write-Error $logMessage;}
        "Warning" {Write-Warning $logMessage;}
        "Debug" {Write-Debug $logMessage;}
        "Info" {Write-Output $logMessage;}

    }
}

If I run the script, this is the output:

PS > .\CallingScript.ps1
Debug preference: Continue
DEBUG: Checking that debugging works
Debug preference: SilentlyContinue
PS >

Answer 1

As JPBlanc's link in his comment explains: It is a variable scope issue. The module's scope chain goes directly to the global scope and not through any script scopes. Even if it is imported from a script.

Your code will work if you set $DebugPreference from your script in the global scope, but of course this has influence on more than just your script.

$global:DebugPreference = "Continue"

Another solution in this specific $DebugPreference case is to use the -Debug parameter to pass it along. The downside is that you will have to do this with every command you call.

Write-Log "Debug" "Checking that debugging still works!" -debug:$DebugPreference

A third solution would be to set $DebugPreference at the module level.

$m = Import-Module Logging -PassThru
& $m {$script:DebugPreference = 'Continue'}