PowerShell string default parameter value does not work as expected

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 workif ($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!

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

seeing many equality comparisons with [String]::Empty, you could use the [String]::IsNullOrWhiteSpace or [String]::IsNullOrEmpty static methods, like the following:

param(    [string]$parameter = $null)# we know this is false($null -eq $parameter)[String]::IsNullOrWhiteSpace($parameter)[String]::IsNullOrEmpty($parameter)('' -eq $parameter)("" -eq $parameter)

which yields:

PS C:\...> .\foo.ps1FalseTrueTrueTrueTrue