PowerShell $null is not null any more when calling into C# code

Question

In PowerShell one can define C# code and execute it. Passing in $null into the following simplest function shows that not null gets passed into the function

Answer 1

Evidently the solution is "don't pass $null to a [String] parameter if you want a $null [String]". There is a special [NullString] class that should be used instead.

PS > [foo]::fooo($Null)
Not Null. Is ''
PS > [foo]::fooo([NullString]::Value)
PS >

I can't find any mentions of the need to use [NullString] in the PowerShell documentation, nor does the source code for the NullString class include any comments explaining its purpose.