Testing Powershell Classes

January 28, 2016
powershell v5 classes

It’s been a busy few months. I recently changed jobs and one of my first projects is building some new custom DSC Resources. I chose to use Class-based resources, because Classes are cool. There were some bumps along the way, one of them being how do you run Unit Tests on such a thing.

Part of the reason I went with a Class-based resource was because it would be easy to test. Do the methods return what I expect? Do the properties have the value I expect?

I quickly discovered that importing a Module does not make any of the types in the module available. If I can instantiate an instance of my class, how can I test it? I poked around Export-ModuleMember, New-ModuleManifest and Add-Type and didn’t see any new option that would export my Type. The only option I found was to execute the Class definition as a script in the current session. So, I came up with something like this Invoke-Expression (Get-Module myClassType).Definition, but if you have more than just a class definition in your module, that could get messy.

Thanks to a suggestion from Doug Finke, I found, what I assume, is the officially supported way to load Powershell Classes.

try the using keyword using 'c:\a\b\target.psm1

There no mention of this feature in the release note and there’s no help for it. Searching the Interwebs only returned one useful page.

PowerShell v5 ‘using namespace’ syntax test

It seemed promising so I tried it, but no luck. However, Doug’s suggestion and the Gist from Altrive suggest the feature is new and maybe was very recently in development. I happened to have download the v5 RTM before it was pulled from the Download Center. BINGO! The syntax using module doesn’t even fully work in the Production Preview.

How To Use “Using”

Here’s an example class just in case you haven’t read anything about the new v5 feature.

# Save as myClassType.psm1
class myType
   [ValidateSet("val1", "Val2")]
   [string] $P1

   static [hashtable] $P2

   hidden [int] $P3

   myType ([string] $s)
       $this.P1 = $s       

   static [void] MemberMethod1([hashtable] $h)
       [myType]::P2 = $h

   [int] MemberMethod2([int] $i)
       $this.P3 = $i
       return $this.P3

Create a module manifest with the RootModule being the psm1 file containing your class(es).

New-ModuleManifest -Path myClassType.psd1 -Author "Chris Hunt" -RootModule myClassType.psm1 -PowerShellVersion 5.0 


Powershell Using

You can see myType isn’t recognized even after I import the myClassType module. However, the using keyword loads it up just like you’d expect from Add-Type.

If you use using in a script, like a Pester script, it has to come before any other statements.

But Wait, There’s More

Again, following the hint from Altrive’s Gist, you can see several options for this keyword.

PS> [Enum]::GetNames('System.Management.Automation.Language.UsingStatementKind')

Cool stuff for a feature that is flying under the radar so far.

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