Get the argument completers that are registered in the current session (if any)

Get-ArgumentCompleter.ps1

function Get-ArgumentCompleter {
<#
.SYNOPSIS
    Get the argument completers that are registered in the current session (if any)
.EXAMPLE
    Import-Module PowerShellGet
    Get-ArgumentCompleter

    shows the argument completers that are registered from the PowerShellGet module
.EXAMPLE
    Import-Module AWS.Tools.Common
    Get-ArgumentCompleter -Name Region

    show the argument completer for the Region parameter that is enforced by default by the AWS module
#>
[OutputType([System.Collections.Generic.KeyValuePair[string,scriptblock]])]
[CmdletBinding()]
param (
    [string]$Name = '*'
)

# the goal here is to get this:
# $ExecutionContext._invokeCommand._context.CustomArgumentCompleters

# But the problem is that the _invokeCommand and _context are Private fields
# which are not accessible through Get-Member, so we need to use Reflection.

# get the total number for all binding flags, we'll need this to get the fields through reflection
$AllFlags  = [System.Enum]::GetValues([System.Reflection.BindingFlags]) -join ','
$BindFlags = ([System.Reflection.BindingFlags]$AllFlags).value__

# first we'll get the private field _invokeCommand and its value
$IcmField = $ExecutionContext.GetType().GetField('_invokeCommand',$BindFlags)
$IcmValue = $IcmField.GetValue($ExecutionContext)

# then we'll get the field _context from the _invokeCommand
$CtxField = $IcmValue.GetType().GetField('_context',$BindFlags)
$CtxValue = $CtxField.GetValue($IcmValue)

# now we want the CustomArgumentCompleters property from _context
$CAPProp  = $CtxValue.GetType().GetProperty('CustomArgumentCompleters',$BindFlags)
$CAPValue = $CAPProp.GetValue($CtxValue)

# finally show the output
$CAPValue.GetEnumerator() | where Key -like $Name
}