PowerShell cmdlet parameter value tab completion

Question

How do you implement the parameter tab completion for PowerShell functions or cmdlets like Get-Service and Get-Process in PowerShell 3.0?

I realise ValidateSet works

Answer 1

I puzzled over this for a while, because I wanted to do the same thing. I put together something that I'm really happy with.

You can add ValidateSet attributes from a DynamicParam. Here's an example of where I've generated my ValidateSet on-the-fly from an xml file. See the "ValidateSetAttribute" in the following code:

function Foo() {
    [CmdletBinding()]
    Param ()
    DynamicParam {
        #
        # The "modules" param
        #
        $modulesAttributeCollection = new-object -Type System.Collections.ObjectModel.Collection[System.Attribute]

        # [parameter(mandatory=...,
        #     ...
        # )]
        $modulesParameterAttribute = new-object System.Management.Automation.ParameterAttribute
        $modulesParameterAttribute.Mandatory = $true
        $modulesParameterAttribute.HelpMessage = "Enter one or more module names, separated by commas"
        $modulesAttributeCollection.Add($modulesParameterAttribute)    

        # [ValidateSet[(...)]
        $moduleNames = @()
        foreach($moduleXmlInfo in Select-Xml -Path "C:\Path\to\my\xmlFile.xml" -XPath "//enlistment[@name=""wp""]/module") {
            $moduleNames += $moduleXmlInfo.Node.Attributes["name"].Value
        }
        $modulesValidateSetAttribute = New-Object -type System.Management.Automation.ValidateSetAttribute($moduleNames)
        $modulesAttributeCollection.Add($modulesValidateSetAttribute)

        # Remaining boilerplate
        $modulesRuntimeDefinedParam = new-object -Type System.Management.Automation.RuntimeDefinedParameter("modules", [String[]], $modulesAttributeCollection)

        $paramDictionary = new-object -Type System.Management.Automation.RuntimeDefinedParameterDictionary
        $paramDictionary.Add("modules", $modulesRuntimeDefinedParam)
        return $paramDictionary
    }
    process {
        # Do stuff
    }
}

With that, I can type

Foo -modules M

and it will tab-complete "MarcusModule" if that module was in the XML file. Furthermore, I can edit the XML file and the tab-completion behavior will immediately change; you don't have to re-import the function.