PowerShell Pester Testing for Parameter Validation

This error caught me out. I am putting this post here firstly to remind me if I do it again adn also to help others who may hit the same issue.

Today I am rewriting a function to create a Hyper-V VM so that I can properly script the creation of my labs for demos and other things. I am doing this because I want to use DSC to create an availability group and want to be able to tear down and recreate the machines (but thats for another day)

I also have been looking at Pester which is a framework for running unit tests within PowerShell

You will find some good blog posts about starting with Pester here

Here is the start of the function. I validate the VMName parameter to ensure that there a VM with that  name does not already exist

function Create-HyperVMFromBase { 
[cmdletbinding()] 
param (
 [Parameter(Mandatory = $true,HelpMessage="Enter a VMName for the VM that does not exist")] [ValidateScript({(!(Get-VM -Name $_))})] 
[string]$VMName,

and my Pester test looks like this

$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path).Replace(".Tests.", ".")
. {'$here\$sut'}

Describe "Create Hyper V from Base Tests" {
    Context "Parameter Values,Validations and Errors" {
        It exists {
        test-path function:\create-hypervmfrombase | should be $true
        }
        It "Should error when VMName exists" {
        $VMName = (Get-VM|Select -First 1 Name).Name
        create-hypervmfrombase -VMName $VMName |should throw
        }

I thought that what I was testing was that the function threw an error when an incorrect parameter was passed. The should throw should be true but what I got was

pester error3

So I was getting the correct error but not passing the test. It was a simple fix. Simply adding curly braces around the call to the function

$here = Split-Path -Parent $MyInvocation.MyCommand.Path
$sut = (Split-Path -Leaf $MyInvocation.MyCommand.Path).Replace(".Tests.", ".")
. "$here\$sut"
Describe "Create Hyper V from Base Tests" {
    Context "Parameter Values,Validations and Errors" {
        It exists {
        test-path function:\create-hypervmfrombase | should be $true
        }
        It "Should error when VMName exists" {
        $VMName = (Get-VM|Select -First 1 Name).Name
        {create-hypervmfrombase -VMName $VMName} |should throw
        }
    }
}

and we pass the test.

pester success2

Advertisements

One thought on “PowerShell Pester Testing for Parameter Validation

  1. Pingback: Parameter Validation Using Pester – Curated SQL

Please feel free to comment on this post. All comments are moderated first before appearing on the site

This site uses Akismet to reduce spam. Learn how your comment data is processed.