Check for Active Directory User in Function Parameters

I’m not sure what it is, but this might’ve been the longest I’ve gone without writing. It’s not that I’ve stopping thinking PowerShell, it’s that I’m so consumed, I’ve had a hard time focusing. My wife calls it my adult ADD. It was just last week that I attended my second PowerShell + DevOps Global Summit (that’s the new name). I learned a great deal and I had a great time. Thanks again to for the contest and free 4-day pass, my employer for picking up the flight, lodging, and food, my parents for their help, and of course to my wife! The weather in Bellevue, WA was perfect and the summit was a huge success. Much gratitude to all those involved in the planning and execution.

It wasn’t long ago that I had a post about comparing Active Directory group memberships. I wanted a pleasant way to visualize and compare the group memberships of two AD users. We had a ticket earlier today, at the office, where I went back to that post and gathered the commands, so I could use them again. I knew I’d need it, I just didn’t know I’d need it so soon. I sent the output results as an image to a coworker, and it wasn’t but a few seconds later that he was interested in what was used to gather that output: it was my modified compare. Here’s an edited version of the image I sent. This image differs from the original post in that I added the users’ SamAccountNames as the property names in my collection. This is why they’re blurred out this time around.


I mentioned to him that I’d write the commands into a wrapper function. While that’s nearly done, I asked myself this question: When should I check if the users exist in Active Directory? Do I do it as part of the function (inside the Begin block, for instance), or do I check at the time the parameter is accepted, as part of a ValidateScript validation?

I’m here today to say that making it a part of the ValidateScript validation attribute is acceptable. While it has the potential to cause an error condition and dump red text to the console, that’s fine. People good at Windows PowerShell read these errors — they’re PowerShell’s errors, and we’re just allowing them to occur. While I can catch these in the function, PowerShell doesn’t always do this for us natively, so I as I see it, it’s not always necessary. Knowing the audience for your tool can help you with this determination. Do you want them to learn PowerShell and get comfortable with the native errors, or does it make sense to protect them from seeing these? It’s up to you. I’m okay with allowing my tools to display some of the native errors, especially since we’re talking about my team using these tools, and not a lower-tiered group. Again, you have to consider who will use your tools, and how they’ll handle the red text.

The test function I wrote, and some error results, can been seen below. In my mind, there were two error possibilities: the computer where the function is invoked doesn’t have the ActiveDirectory module, or one, or both, of the users don’t exist in Active Directory. Again, in many cases, these standard PowerShell errors can be delivered without the need to hide them from the user of the function. Here’s a test function now.

Function Test-ADUser {
    Param (
        [ValidateScript({Get-ADUser -Identity $_})]

        [ValidateScript({Get-ADUser -Identity $_})]

    Write-Output -InputObject "Success on both parameters."

This first image below, shows the error condition when the ActiveDirectory module isn’t available on the system where the function is invoked. It displays the standard, I-don’t-recognize-what-you-entered error: “Get-ADUser is not recognized…”


In this image, we can see the error message that occurs when one of the supplied users doesn’t exist in Active Directory. It won’t get this far unless the ActiveDirectory module is on the system where this function is invoked.


So in the end, consider when writing your tools, if a standard PowerShell error is acceptable or not. I think that there’s times it’s suitable, depending on the error and the group using the tool.

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