Using 'Pester' (PowerShell Unit-Testing framework) with properties

Question

I am trying to wrap a text fixture around some PowerShell code that extends an object with a property. I get an error that appears to be caused by Pester. I have a contrived example below that displays what I am trying to do.

Has anyone succeeded in writing tests on functions that use properties with Pester?

The error I get:

Describing Get-PropertyOfItem
Select-Object : Property cannot be processed because property "should" already exists.
At C:\Repos\ClinicientOps\clinicientops\General\Functions\Get-PropertyOfItem.ps1:4 char:11
+     $files | Select-Object *, @{Name = "TestProperty"; Expression = { $dir.Length}} ...
+              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (Windows:PSObject) [Select-Object], PSArgumentException
    + FullyQualifiedErrorId : AlreadyExistingUserSpecifiedPropertyNoExpand,Microsoft.PowerShell.Commands.SelectObjectCommand

My function:

function Get-PropertyOfItem {
    $dir = "C:\"
    $files = Get-ChildItem $dir
    $files | Select-Object *, @{Name = "TestProperty"; Expression = { $dir.Length}} -Last 1
}

My test code:

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

Describe "Get-PropertyOfItem" {

    It "does something useful" {

        $prop = Get-PropertyOfItem
        $prop.TestProperty.should.be(3)
    }
}

From scottmuc on pester's github: "Bah, seems like the object extensions are causing more issues. We're looking at removing the $object.should extension and using pipeline based assertions for version 2.0 which should fix this." github.com/pester/Pester/issues/33 — msgisme
– msgisme, Commented Nov 30, 2012 at 20:02

Peter Mortensen · Accepted Answer · 2018-12-31 12:13:33Z

1

It appears to be a limitation they are investigating in version 2.

edited Dec 31, 2018 at 12:13

Peter Mortensen

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answered Nov 30, 2012 at 20:04

msgisme

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1 Comment

Peter Mortensen Over a year ago

Can you add the context - a reference to the GitHub content?

Peter Mortensen · Accepted Answer · 2018-12-31 12:15:14Z

1

Pester version 2.0.1 has been silently released. You'll have to rewrite your expectation to be

$prop.TestProperty | Should Be 3

It also means that all your other tests will need to migrate to this pipeline form Expectation syntax.

edited Dec 31, 2018 at 12:15

Peter Mortensen

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answered Feb 3, 2013 at 15:24

Scott Muc

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2 Comments

msgisme Over a year ago

I'm having a really hard time getting started with Pester. I am a DBA who is pretty fresh to BDD. Most of the testing I need to do is either environment specific (make sure the environment is configured) or integrating multiple steps. The wiki seems to focus on testing individual functions. Do you have any suggestions for learning how to use this tool?

msgisme Over a year ago

One thing that would help is a page explaining how to setup Pester- I'm curious when I follow the directions here: scottmuc.com/blog/development/…... what version do I get, 1.2 or 2.0?

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