Inconsistent behavior in powershell with null parameters - powershell

Question

I need to write a function in powershell that tells apart a 'parameter not being passed' from one passed with string empty (or any other string)
I wrote it like this:
function Set-X {
param(
[AllowNull()][string]$MyParam = [System.Management.Automation.Language.NullString]::Value
)
if ($null -ne $MyParam) { write-host 'oops' }
else { write-host 'ok' }
}
If I call Set-X without parameters from ISE, it works as I expect and prints 'ok'.
But if I do that from the normal console, it prints 'oops'.
What is going on? What is the proper way to do it?

Answer 1

Allowing the user to pass in a parameter argument value of $null does not change the fact that powershell will attempt to convert it to a [string].
Converting a $null value in powershell to a string results in an empty string:
$str = [string]$null
$null -eq $str # False
'' -eq $str # True
(same goes for $null -as [string] and "$null")
Remove the type constraint on the MyParam parameter if you not only want to allow $null but also accept $null as a parameter value:
function Set-X {
param(
[AllowNull()]$MyParam = [System.Management.Automation.Language.NullString]::Value
)
if ($null -ne $MyParam) { write-host 'oops' }
else { write-host 'ok' }
}

Answer 2

As Mathias and BenH have written, the culprit is casting $null to the [string] type, which results in an empty string:
[string]$null -eq '' #This is True
But for the sample code in Mathias answer to work correctly we also have to replace
[System.Management.Automation.Language.NullString]::Value
with $null
function Set-X {
param(
[AllowNull()]$MyParam = $null
)
if ($null -ne $MyParam) { write-host 'oops' }
else { write-host 'ok' }
}