Unit Testing within Modules

Introduction

If you have code like this inside a a PS module (.psm1 file):

function BuildIfChanged {
    $thisVersion=Get-Version
    $nextVersion=Get-NextVersion
    if($thisVersion -ne $nextVersion) {Build $nextVersion}
    return $nextVersion
}

function Build ($version) {
           write-host "a build was run for version: $version"
}
#...other functions not included

Export-ModuleMember BuildIfChanged

The "problem" here is that the only function that's exposed in the public space so to say is the BuildIfChanged function. BUT, we need to be able to mock the other functions to be able to write tests like this:

Describe "BuildIfChanged" {
    Context "Wnen there are Changes" {
        Mock Get-Version {return 1.1}
        Mock Get-NextVersion {return 1.2}
        Mock Build {} -Verifiable -ParameterFilter {$version -eq 1.2}

        $result = BuildIfChanged

        It "Builds the next version" {
            Assert-VerifiableMocks
        }
        It "returns the next version number" {
            $result.Should.Be(1.2)
        }
    }
    Context "When there are no Changes" {
        Mock Get-Version -MockWith {return 1.1}
        Mock Get-NextVersion -MockWith {return 1.1}
        Mock Build -MockWith {}

        $result = BuildIfChanged

        It "Should not build the next version" {
            Assert-MockCalled Build -Times 0 -ParameterFilter{$version -eq 1.1}
        }
    }
}

Oh, and btw we don't want to/can extract those 'helper' functions into their own files (which could be a solution).

Solution/Workaround

A simple workaround is to "fake" dot sourcing...which doesn't work for .psm1 files OOB with PowerShell. At the top of the test file we’ll put this snippet of code:

$module = (Split-Path -Leaf $PSCommandPath).Replace(".Tests.ps1", ".psm1")
$code = Get-Content $module | Out-String
Invoke-Expression $code

Downsides? Yes, there’s one at least and that is you can have trouble debugging since you’re no longer using the “real” module but just the contents of it so to say. I haven’t found a way to debug code that has been put into a script using Invoke-Expression at least.