Unexpected behavior of AutomationNull in object initializers (hashtable or [pscustomobject])

[mklement0]

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Steps to reproduce

In an array-type-constrained variable (as in expressions in general), AutomationNull is treated like $null; e.g.:

# Note: & { } is a simple way to return the AutomationNull singleton.
# OK: no error, and $null is assigned.
[string[]] $arr = & { }

By contrast, using AutomationNull in hashtable / [pscustomobject]-based object initializers fails:

class Example {
   [string[]]$LogMessage
}

$automationNull = & {}

# !! BREAKS: "Unable to cast object of type 'System.Management.Automation.PSObject' to type 'System.String[]'"
[Example] @{ 'LogMessage' = $automationNull }

# !! BREAKS differently: ""Type 'System.String[]' does not have a default constructor (Parameter 'type')""
[Example] [pscustomobject] @{ 'LogMessage' = $automationNull }

That the [pscustomobject]-based initialization breaks differently is surprising, and reminiscent of #19384 (comment)

Expected behavior

Two [Example] instances should be constructed, with $null in the .LogMessage property.

Actual behavior

Both initializations fail, albeit differently:

InvalidArgument: /path/to/script.ps1:13
Line |
  13 |  [Example] @{ 'LogMessage' = $automationNull }
     |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Cannot create object of type "Example". Unable to cast object of type 'System.Management.Automation.PSObject' to type 'System.String[]'.
InvalidArgument: /path/to/script.ps1:16
Line |
  16 |  [Example] [pscustomobject] @{ 'LogMessage' = $automationNull }
     |  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     | Cannot convert value "@{LogMessage=}" to type "Example". Error: "Type 'System.String[]' does not have a default constructor (Parameter 'type')"

Error details

See above.

Environment data

PowerShell 7.4.0-preview.2

Visuals

No response

[SteveL-MSFT]

The first case should treat AutomationNull the same as Null which would result in an empty string. The second case is completely separate due to pscustomobject type conversion.

[mklement0]

@SteveL-MSFT, the second case may be technically distinct, but it should behave the same and is equally broken.

[ghost]

🎉This issue was addressed in #19415, which has now been successfully released as v7.4.0-preview.3.:tada:

Handy links:

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