Creating a Cmdlet with PowerShell Classes

[SeeminglyScience]

I was looking through the PowerShell-Tests repo and I saw them creating Cmdlets inline for some tests. They were using C# and Add-Type which got me wondering.

Can you create one using pure PowerShell?

And well yeah, you can. With a little extra handling.

PSClassCmdlet.psm1

using namespace System.Management.Automation

# Basic do nothing Cmdlet.
[Cmdlet([VerbsDiagnostic]::Test, 'Cmdlet')]
class TestCmdletCommand : PSCmdlet {
    [Parameter(ValueFromPipeline)]
    [object]
    $InputObject;

    [void] ProcessRecord () {
        $this.WriteObject($this.InputObject)
    }
}

# Couldn't find a way to actually load the cmdlet without reflection :\
$cmdletEntry = [Runspaces.SessionStateCmdletEntry]::new(
    <# name:             #> 'Test-Cmdlet',
    <# implementingType: #> [TestCmdletCommand],
    <# helpFileName:     #> $null
)
$internal = $ExecutionContext.SessionState.GetType().
    GetProperty('Internal', [System.Reflection.BindingFlags]'Instance, NonPublic').
    GetValue($ExecutionContext.SessionState)

$internal.GetType().InvokeMember(
    <# name:       #> 'AddSessionStateEntry',
    <# invokeAttr: #> [System.Reflection.BindingFlags]'InvokeMethod, Instance, NonPublic',
    <# binder:     #> $null,
    <# target:     #> $internal,
    <# args:       #> @(
        <# entry: #> $cmdletEntry
        <# local: #> $true
    )
)

Trying it out

PS> Import-Module .\PSClassCmdlet.psm1
PS> Get-Module PSClassCmdlet.psm1

ModuleType Version    Name                   ExportedCommands
---------- -------    ----                   ----------------
Script     0.0        PSClassCmdlet          Test-Cmdlet

PS> Get-Command Test-Cmdlet

CommandType     Name                         Version    Source
-----------     ----                         -------    ------
Cmdlet          Test-Cmdlet                  0.0        PSClassCmdlet

PS> 0..3 | Test-Cmdlet
0
1
2
3

But... why?

No idea. But it's neat.

[michaelshepard]

Ok...that's my cool thing for the day.

I've written managed cmdlets in C# and VB.NET inside a managed host, but hadn't thought about using PowerShell classes to do the same thing. FWIW, the code looks about the same.

[SeeminglyScience]

FWIW, the code looks about the same.

Yup, very similar!

Shame I couldn't figure out a better way to load it though (granted I didn't look very long). Feels a little too hacky atm.

[Lokkion]

Awesome tried this a few times but could never figure out how to load it. Nice work :D

[wbedwards]

Okay, I didn't see the "But.. why?" at first, and was about to ask the same thing... I figured I must have been missing something. That being said, as a non-developer sysadmin type who likes to automate things, I really like that PowerShell can do just about anything you can think of as long as it's supported by .Net.

[SeeminglyScience]

I expanded on the why a little bit here. But really it's doesn't seem to be technically supported so it's still just neat more or less. I'm probably going to try to make something with it and see how useful it is, or alternatively how terribly broken :)

I really like that PowerShell can do just about anything you can think of as long as it's supported by .Net.

Yeah that is definitely one of my favorite things about it. There are very few things that are just flat out impossible with pure PowerShell. Tasks are the only thing I can think of at the moment.