Creating a Class based DSC Resource using PowerShell

Jacob Benson

Scenario: I would like to think I am fairly competent at creating DSC Resources using .MOF files. But with the upcoming release of Windows Management Framework 5.0 we can now write DSC Resources using Classes. I have never done anything with a Class. I am going to attempt to figure out how to write a simple Class based DSC Resource.

I am going to start with this TechNet Article, because it was the first thing that came up when I Googled “Create Class based DSC Resource”.

I am going to create a DSC Resource called MyTestClassResource. The first thing I need to do is create the appropriate folder structure, which I do by running the following two commands:

Next, I need to create the Module Manifest.

The .PSM1 file is where I define and create my Class based resource.

Now, let’s get to creating this Resource. I am going to go super simple here and create a Resource that will just ensure that a Folder exists. Yes, I know the File DSC Resource already does this, that’s not the point :). After fiddling around for a little bit here is what my Class DSC Resource “Skeleton” looks like.

The rest of this should be pretty straightforward. Right? The first thing I try to do in the Get section is to test and see if the path exists.

This doesn’t work however because “Variable is not assigned in the method”, whatever the hell that means. Looking at the article it uses a $This object (I have no idea if that’s even the right word) with the variables to do things, so let’s try a different tactic.

And that works fine, so clearly $This is some special thing I need to be using going forward. This should be fun :). Now that I got that working, the rest of this Method was pretty straight forward.

After that, the Test Method is pretty easy as well. However, I have no idea what is going on with the whole Return -not $Item part, I am just following along from the example and hoping that it works.

Onto the Set Method. One thing I noticed when creating this is that I don’t need an Else block with an If statement, which is nice.

With that done I need to re-create the Module Manifest that I made earlier with some important information.

At this point while trying to run Get-DSCResource I realized that my folder structure I created at the beginning was not correct, because I was used to the way I had been doing it when working with .MOF Files. I actually need only this:

And then I moved the .psm1 and .psd1 files into that folder. No extra sub-folder required. This is a win in my book.

Now, when I run Get-Module -ListAvailable, the MyTestClassResource Module is listed. However, when I run Get-DSCResource -Module MyTestClassResource I get a lot of nothing. Weird. Next I try to Import-Module -Name MyTestClassResource and I get this giant bundle of joy.

Uhhh…What? Thinking quickly I decide that it’s probably not going to work to name everything exactly the same. I rename the MyTestClassResource folder to MyTestClassResources and leave everything else the same. And well, I will spare all the error text but that didn’t work at all either. I am not sure how much time I spent trying to figure out what the hell I was doing wrong, but to say it was an exercise in frustration is a massive understatement. No matter what I did I couldn’t get it to recognize a valid Module, let alone DSC Resource. I lost track of all the troubleshooting that I did but here is what my folder structure looks like now that it is working. Note that I renamed my Class based Resource from MyTestClassResource to MyFolderResource.
ClassTest

Now, let’s test a Configuration!

And just to make sure it’s working, let’s set it to Absent