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

Add-Type -TypeDefinition @"
public static class foo
{
public static void fooo(string a)
{
    if(a!=null)
    {
        System.Console.WriteLine("Not Null. Is '" + a + "'");
    }
}
}
"@ -Language CSharp

Calling it as follows leads to the output Not Null. Is ''. This shows that $null was not null in C#. Methods like 'IsNullOrEmpty' or 'IsNullOrWhiteSpace' return true, so PowerShell must have implicitly converted the $null into a string.

[foo]::fooo($Null)

Any ideas why this is happening and if there is a better fix apart from rather calling String.IsNullOrEnpty in C#? NB: This happens irrespective of the C# language specified 'Add_Type'. I'm using PowerShell V5.

[foo]::fooo([String] $null), [foo]::fooo($null -as [String]), and $s = $null; [foo]::fooo($s) all yield the same result. If the type of parameter a is changed from string to object then fooo no longer prints any output. That suggests PowerShell is trying to be "smart" in its handling of string parameters. — Lance U. Matthews
– Lance U. Matthews, Commented Aug 15, 2016 at 16:39
The same behavior can be observed with the new class support in PowerShell 5.0 (class foo { static fooo([String] $a) { if ($a -ne $null) { [Console]::WriteLine("Not Null. Is '" + $a + "'"); } } }; [foo]::fooo($null);) as well as plain old PowerShell functions (function fooo([String] $a) { if ($a -ne $null) { [Console]::WriteLine("Not Null. Is '" + $a + "'"); } }; fooo $null;), so this must be how PowerShell handles string parameters in general. — Lance U. Matthews
– Lance U. Matthews, Commented Aug 15, 2016 at 16:51
Possible duplicate of Possible to pass null from Powershell to a .Net API that expects a string? — Lance U. Matthews
– Lance U. Matthews, Commented Aug 16, 2016 at 18:36
Thanks for your investigation. Seems like this is just PowerShell trying to be overly helpful. I knew that PowerShell sometimes casts objects without asking to be 'helpful' but I did not expect it to actually instantiate it an object. I suppose using String.IsNullOrEmpty() in C# code is the best solution to deal with this problem. — bergmeister
– bergmeister, Commented Aug 17, 2016 at 23:30

Lance U. Matthews · Accepted Answer · 2016-08-15 17:17:05Z

6

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.

answered Aug 15, 2016 at 17:17

Lance U. Matthews

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1 Comment

Jeppe Stig Nielsen Over a year ago

See documentation NullString Class. This possibility was new in PowerShell 3 (since 2012).

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