PowerShell string default parameter value does not work as expected

Question

#Requires -Version 2.0  [CmdletBinding()] Param(   [Parameter()] [string] $MyParam = $null )  if($MyParam -eq $null) {   Write-Host 'works' } else {   Write-Host 'does not work' } 

Outputs "does not work" => looks like strings are converted from null to empty string implicitly? Why? And how to test if a string is empty or really $null? This should be two different values!

Answer 1

Okay, found the answer @ https://www.codykonior.com/2013/10/17/checking-for-null-in-powershell/

Assuming:

Param(   [string] $stringParam = $null ) 

And the parameter was not specified (is using default value):

# will NOT work if ($null -eq $stringParam) { }  # WILL work: if ($stringParam -eq "" -and $stringParam -eq [String]::Empty) { } 

Alternatively, you can specify a special null type:

Param(   [string] $stringParam = [System.Management.Automation.Language.NullString]::Value ) 

In which case the $null -eq $stringParam will work as expected.

Weird!

Answer 2

You will need to use the AllowNull attribute if you want to allow $null for string parameters:

[CmdletBinding()] Param (     [Parameter()]      [AllowNull()]     [string] $MyParam ) 

And note that you should use $null on the left-hand side of the comparison:

if ($null -eq $MyParam) 

if you want it to work predictably