How can I know that PowerShell function parameter is omitted

Consider such function:

function Test($foo, $bar)
{
  ...
}

We can call it:

Test -foo $null
Test

How can I know when the -foo was omitted, and when it was $null?

From stackoverflow

• Unless it is possible to trap exceptions thrown from param statement (and since param has to be the first, I can't see this would work):

  function {
    trap { "Something failed" }
    param($foo = $(throw "Foo not specified"))
    ...
  

  Then I can't see a way (you get the same thing with [switch] parameters: default or explicitly -mySwitch:$false).

• The solution based on Richard's idea:

  $missed = "{716C1AD7-0DA6-45e6-854E-4B466508EB96}"
  
  function Test($foo = $missed, $bar)
  {
      if($foo -eq $missed) {
          Write-Host 'Missed'
      }
      else
      {
          Write-Host "Foo: $foo"
      }
  }
  
  Test -foo $null
  Test
  

• If you are using Powershell V2, you can use the $PSBoundParameters variable which is a dictionary that lists all bound parameters at current scope.

  See this blog post that discusses it.