Example of ArgumentCompleter looking back in Command AST

DetectPreviousCommandArgCompleter.ps1

# See here for question: https://rohnspowershellblog.wordpress.com/2017/01/17/completing-parameter-values-with-other-parameter-values/#comment-437

function Test-DetectProperty {
<#
.SYNOPSIS
Proof of concept to show that inspecting preceding command elements is possible
from within ArgumentCompleters, at least in a limited way.

.EXAMPLE
Get-Service | Test-DetectProperty -Property <PressTab>

.EXAMPLE
$Services = Get-Service
$Services | Test-DetectProperty -Property <PressTab>

# Note that this won't work b/c we we don't handle the particular AST type:
$Services[0] | Test-DetectProperty -Property <PressTab>

# This won't work either, b/c there are two commands you'd have to walk back:
$Services | Select-Object -First 1 | Test-DetectProperty -Property <PressTab>

# This will, though:
$SingleService = $Services | Select-Object -First 1
$SingleService | Test-DetectProperty -Property <PressTab>
#>
    [CmdletBinding()]
    param(
        [Parameter(ValueFromPipeline)]
        $InputObject,
        [ArgumentCompleter({
            param($commandName, $parameterName, $wordToComplete, $commandAst, $fakeBoundParameter)

            $Pipeline = $commandAst.Parent
            $PrecedingElement = $null
            $FoundPrecedingElement = $fase
            foreach ($Element in $Pipeline.PipelineElements) {
                if ($Element -eq $commandAst) {
                    $FoundPrecedingElement = $true
                    break
                }
                $PrecedingElement = $Element
            }

            if (-not $FoundPrecedingElement) {
                $PrecedingElement = $null
            }

            $Properties = if ($PrecedingElement) {
                switch ($PrecedingElement.GetType().Name) {
                    CommandAst { # Another command
                        $PrevCommandName = $PrecedingElement.CommandElements[0].ToString()
                        $CmdInfo = Get-Command $PrevCommandName -ErrorAction SilentlyContinue
                        if ($CmdInfo.OutputType) {
                            $CmdInfo.OutputType.Type.GetProperties().Name | Sort-Object -Unique
                        }
                    }

                    CommandExpressionAst { # Expression
                        if ($PrecedingElement.Expression.GetType().Name -eq 'VariableExpressionAst') {
                            $PrecedingVar = Get-Variable $PrecedingElement.Expression.VariablePath.ToString() -ErrorAction SilentlyContinue -ValueOnly
                            if ($PrecedingVar) {
                                $PrecedingVar | Get-Member | ForEach-Object Name
                            }
                        }
                    }
                }
                
            }
            if ($Properties) {
                $Properties.ForEach{
                    [System.Management.Automation.CompletionResult]::new($_, $_, 'ParameterValue', $_)
                }
            }
        })]
        [string[]] $Property
    )

    process {
        $PSBoundParameters
    }
}

Very Nice

If you change:

    $PrecedingVar | Get-Member | ForEach-Object Name

Into:

   $PrecedingVar | Get-Member -MemberType NoteProperty | ForEach-Object Name

You get only the NotePropertyNames

Realy handy