Jakub Jareš

Pester5 - discovery and script setup

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As described in the v5 readme, the fundamental change in Pester5 is that Pester now runs in two phases: Discovery and Run. During Discovery Pester finds all your tests, groups them together and filters them based on your filters. Then in the Run phase it will actually run them.

Splitting the work into two distinct phases, powers many of the features in this release, and enables many others to be implemented in the future.

For Discovery to work correctly, there are new rules to follow:

Put all your code into It, BeforeAll, BeforeEach, AfterAll or AfterEach. Put no code directly into Describe, Context or on the top of your file, without wrapping it in one of these blocks, unless you have a good reason to do so.

All misplaced code will run during Discovery, and its results won’t be available during Run.

This article offers more guidance and shows examples of what it means.

Pester5 - importing your ps1 files

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In Pester 5 you should put your script setup inside of a BeforeAll block. If you are still using this historical approach, then it will no longer work:

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

Describe "Get-Emoji" {
    It "Gets cactus" {
        # your test code

The shortest way to a WPF App

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I often see a lot of unnecessary code in WPF demos in PowerShell, so I want to share the most barebone version that still works correctly.

You don’t need to use New-Object System.Xml.XmlNodeReader $xaml, and you don’t need [Windows.Markup.XamlReader]::Load($reader). Just use Parse.

You also don’t need the x and d namespaces (most of the time) so you can remove them as well.

Just make sure that the $xaml variable has type [string], and you can get a working app in just few lines of code.

Add-Type -AssemblyName PresentationFramework

[String]$xaml = @"
<Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
        <Label FontSize="100" Name="Text" /> 

$Window = [Windows.Markup.XamlReader]::Parse($xaml)

$Text = $Window.Content.FindName("Text")
$Text.Content = "👋, from WPF!"


Testing self-contained scripts with Pester, part 2

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Few days ago I posted about a tiny module I wrote to skip the entry point function in a script. I got few reactions telling me that there are better ways to organize your scripts, and they were all correct. Putting your code into a module and distributing it that way, or splitting the script into different files and combining them during build are both better than having a single file with everything.

Pester - Using Because in tests

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This weekend I added custom failure reasons into Pester. This feature I know and love from Fluent Assertions, so let me show you how I would use it in Pester.

🔥 This feature is not released yet, get pre-release version 4.2.0-alpha3 to use it.

Because parameter

The new feature adds an optional -Because parameter to all Should assertions. The parameter allows you to specify a reason that will be shown when assertion fails, like so:

Using emojis in PowerShell code

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Few days ago I saw a post about using hieroglyph unicode characters in Haskell to write fully functional haskell code. They shown an example of map function. A function that applies a function to a collection of items. Pretty much how foreach does in PowerShell.

I thought this was fun, and tweeted this:

Testing class based DSC resources with Pester

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I am using PowerShell version 5.0.10018.0 for the examples. This version of PowerShell ships with the WMF February 2015 Preview package that you can download here.

I was asked if there are any resources on testing class based DSC resources. And to be honest I am not sure. We shortly discussed the possibilities on the PS MVP mailing list, but I am still not sure what are the possibilities. So why not discover them while learning something more about the topic.

Luckily there are quite a few great resources on how to actually create a class based DSC resource, and what you need to do that. Some of them are:

Creating a Class based DSC Resource using PowerShell

Class-defined DSC resources in Windows Management Framework 5.0 Preview

Writing a custom DSC resource with PowerShell classes

Why do we test?

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Lately we are getting some great questions on our Pester issue page, I am reposting soem of my answers as blog posts, because I hope they are worth reading. You can access the original question here.

We test because we need a simple set of boundaries that define a more complicated system. Coming up with simple tests and gradually refining them to define more complex systems is easy for us humans. Definitely easier than defining a complex system in a single swoop.

Lowering IO Priority of PowerShell Process

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This week brought quite a few challenges. One of them was a question asked by a friend:

How do I search contents of all the files for given string, without killing the performance of the computer?

This seemed like a simple question to answer: Just lower the priority of the PowerShell process to Idle.

(Get-Process -Id $pid).PriorityClass = 'Idle'

The only problem is, that it does not work.