ConstrainedLanguage mode: [pscustomobject] literals cannot be used, even though the type is white-listed, and an equivalent New-Object call succeeds

[mklement0]

Prerequisites

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Steps to reproduce

# From a regular session (with language mode 'FullLanguage')
# CAVEAT: You won't be able to undo this in the same session.
$ExecutionContext.SessionState.LanguageMode = 'ConstrainedLanguage'
[pscustomobject] @{ foo = 1 }

Perhaps I'm missing something, but the fact that hashtable literals alone (without the [pscustomobject] "cast") work fine, and that [pscustomobject] is among the whitelisted types, as well as the fact that the equivalent New-Object pscustomobject -Property @{ foo = 1 } does work, leads me to believe that this is a bug (it isn't specific to 7.4.0 - I see it in 7.3.8 too, and even in Windows PowerShell).

Expected behavior

A [pscustomobject] instance should be constructed and output.

Actual behavior

An error occurs (see below).

Error details

Curiously, the error message depends on whether at least one such literal was defined earlier, while the session was still in FullLanguage mode:

• In the latter case - note the confusing reference to restricted mode:

InvalidArgument: Cannot convert value "System.Collections.Specialized.OrderedDictionary" to type "System.Management.Automation.LanguagePrimitives+InternalPSCustomObject". 
Error: "Cannot convert hashtable to an object of the following type: System.Management.Automation.LanguagePrimitives+InternalPSCustomObject. 
Hashtable-to-Object conversion is not supported in restricted language mode or a Data section."

• If no [pscustomobject] literal was defined before entering constrained mode:

InvalidArgument: Cannot convert value to type "System.Management.Automation.LanguagePrimitives+InternalPSCustomObject". 
Only core types are supported in this language mode.

Environment data

PowerShell 7.4.0

Visuals

No response

[nightroman]

@mklement0 Thank you for digging into this. One (side) question, please

leads me to believe that this is a bug (it isn't specific to 7.4.0 - I see it in 7.3.8 too, and even in_Windows PowerShell).

What exactly changed in 7.4.0 so that the problem became visible?

[mklement0]

@nightroman, the only thing I'm personally aware of is the presumptive bug in the newly introduced audit mode for the system lockdown feature:

• System lockdown / WDAC: Use of new audit-only mode prevents use of [pscustomobject] literals, behaves as if ConstrainedLanguage mode were in effect #20768

[no-identd]

See also MicrosoftDocs/PowerShell-Docs#6884 & #14071, found via https://stackoverflow.com/questions/64805592/automatically-retrieve-allowed-types-for-constrained-language-mode/64806919#comment114584908_64805592, itself found via https://www.reddit.com/r/PowerShell/comments/10zb95d/pscustomobject_doesnt_work_with_task_scheduler/

@nightroman @mklement0 I think the answers given in the two above issues should address some of the confusion about this voiced here, albeit I haven't had time to read them in detail yet

Stack Overflow

Automatically retrieve Allowed Types for Constrained Language mode

For my hobby project ConvertTo-Expression, I would like the output expression of my cmdlet (by default) compliant with the Constrained Language mode. For this, I might include a hardcoded list with

Blocked

[mklement0]

Thank you, @no-identd.

• While In RestrictedLanguage mode, an instance of a given type cannot be created with a cast, but can be created with New-Object #14071 (which I'd forgotten about) discusses a similar asymmetry, it pertains to restricted language mode, and the only reason no action was taken was there was because "because RestrictedLanguage is used for a different purpose, which no longer exists in PowerShell.".

• Given that this issue is about constrained language mode - which appears to be alive and well - I think the asymmetry is worth fixing (that is, [pscustomobject] literals / casts should be permitted too).