Writing stealth code in PowerShell

What happens in module, stays in module.

Most of my scripts are using Import-Component function to bulk-import dependencies (PS1 files with functions, modules, source code, .Net assemblies).

To import PS1 files with functions, they have to be dot-sourced and that provided me with some challenge: if PS1 is dot-sourced inside the function, it will be available only in that function’s scope. To overcome this, I could scope each contained function, alias, and variable as global (nasty!) or call Import-Component function itself using dot-sourcing (yes, you can dot-source more than just files).

For a while, dot-sourcing Import-Component seemed to work fine, until one day, I realized, that this effectively pollutes caller’s scope with all Import-Component‘s internal variables. Consider this example:

function DotSource-Me
    $MyString = 'Internal variable'

$MyString = 'External variable'

# Calling function as usual
Write-Host "Function was called, 'MyString' contains: $MyString"

# Dot-sourcing function
. DotSource-Me
Write-Host "Function was dot-sourced, 'MyString' contains: $MyString"

If we run this script, the output will be:

Function was called, 'MyString' contains: External variable
Function was dot-sourced, 'MyString' contains: Internal variable

As you can see, when the DotSource-Me function is called as usual, it’s internal variable is restricted to the function’s scope and doesn’t affect the caller’s scope. But when it’s dot-sourced, variable in the caller’s scope is overwritten.

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