Class member is json-serialized as empty string instead of null

Question

I'm using the class keyword in a powershell script.

I want to serialize instances of my custom class, but I observe that $null members are serialized as "" instead of null.

Reproduction:

Class foo{
    [string]$X
}

[foo]@{ x = $null } | ConvertTo-Json

$foo = New-Object foo
$foo.X = $null

$foo | ConvertTo-Json

Outputs :

{
    "X":  ""
}
{
    "X":  ""
}

But I'm expecting :

{
    "X":  null
}
{
    "X":  null
}

As a side note, this works :

@{ X = $null } | ConvertTo-Json

It outputs expected :

{
    "X":  null
}

Is there a way to fix this ?

PS: if it matters $PSVersionTable outputs:

Name                           Value                                                                                                                                                                     
----                           -----                                                                                                                                                                     
PSVersion                      5.1.19041.546                                                                                                                                                             
PSEdition                      Desktop                                                                                                                                                                   
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}                                                                                                                                                   
BuildVersion                   10.0.19041.546                                                                                                                                                            
CLRVersion                     4.0.30319.42000                                                                                                                                                           
WSManStackVersion              3.0                                                                                                                                                                       
PSRemotingProtocolVersion      2.3                                                                                                                                                                       
SerializationVersion           1.1.0.1

No, there's no way to fix that due to the way PowerShell implements typing - assignment to a variable that has been type-cast `[string]` will never result in `$null`, always an empty string — Mathias R. Jessen, Oct 21 '20 at 13:55
If you are able to use 3rd party libraries for this project you could also check out NewtonSoft. I think it's kind of the industry standard in .NET for dealing with json and you get a lot more control of the output — Efie, Oct 21 '20 at 15:21

Mathias R. Jessen · Answer 1 · 2020-10-21T14:41:43.443

Is there a way to fix this ?

No

(a workaround exists depending on the level of control you have over the input, but there's no way to assign $null directly to a [string]-typed PowerShell class property)

Runtime typing enforcement in PowerShell classes doesn't rely on .NET's implicit type conversion rules - this is offloaded to PowerShell's existing language infrastructure and the conversion logic found there instead, and the default type converter for [string] always returns an instance, never null.

You can observe the same behavior when typing variables:

Remove-Variable X -Force

# Let's assign $null to a variable with no type constraints
$X = $null
# True - we can assign $null to a variable without type constraints just fine
$null -eq $X

# Let's apply a type constraint and assign `$null` again
[string]$X = $null
# False - actual value assigned after type constraint applied is not $null anymore
$null -eq $X
# True - PowerShell has converted $null to ''
'' -eq $X

So you have to type $X as something other than a string ([object] for example) if you want it to retain $null values

mklement0 · Accepted Answer · 2020-12-16T17:20:26.943

Mathias R. Jessen's helpful answer explains the problem well: if you assign $null to a [string] typed property or variable, you'll end up with '' (the empty string).

There is an - obscure - workaround, which assumes that you have control over the values you assign:

You can assign [NullString]::Value to a [string]-type-constrained variable or property to make it store a $null value:

Class foo {
  # Default to $null
  [string] $X = [NullString]::Value
}

[foo]::new() | ConvertTo-Json

[foo] @{ X = [NullString]::Value } | ConvertTo-Json

($o = New-Object foo).X = [NullString]::Value
$o | ConvertTo-Json

Note:

The primary purpose of [NullString]::Value is to allow passing true null ($null) values to string-typed parameters of .NET Methods, which wouldn't work when passing [string]-typed PowerShell variables or properties, which on assignment invariably convert $null to '' (the empty string), as explained in Mathias' answer.
While you can use it in pure PowerShell code as well, as shown above, be mindful that other code that accepts [string]-typed values may not expect them to ever be $null, and that passing a value to another [string]-constrained (parameter) variable again converts to ''.
See this answer for more information.

The above yields:

{
  "X": null
}
{
  "X": null
}
{
  "X": null
}

Class member is json-serialized as empty string instead of null

2 Answers2

No