Object[] should cast to IEnumerable[<>]

[iRon7]

Prerequisites

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• Refer to Differences between Windows PowerShell 5.1 and PowerShell.

Steps to reproduce

I am not quiet sure about this, but I would expect PowerShell to be able to cast (CanConvertTo) an object array (Object[]) to a specific IEnumerable. The issue occurred to me when I was trying to create an InvariantCultureIgnoreCase HashSet and immediately populate it in a single statement:

[System.Collections.Generic.HashSet[String]]::new(@('a', 'b', 'c', 'A', 'b'), [StringComparer]::InvariantCultureIgnoreCase)

Expected behavior

a
b
c

Actual behavior

MethodException: Cannot find an overload for "new" and the argument count: "2".

Exception             :
    Type        : System.Management.Automation.MethodException
    ErrorRecord :
        Exception             :
            Type    : System.Management.Automation.ParentContainsErrorRecordException
            Message : Cannot find an overload for "new" and the argument count: "2".
            HResult : -2146233087
        CategoryInfo          : NotSpecified: (:) [], ParentContainsErrorRecordException
        FullyQualifiedErrorId : MethodCountCouldNotFindBest
        InvocationInfo        :
            ScriptLineNumber : 1
            OffsetInLine     : 1
            HistoryId        : -1
            Line             : [System.Collections.Generic.HashSet[String]]::new(@('a', 'b', 'c', 'A', 'b'), [StringComparer]::InvariantCultureIgnoreCase)
            PositionMessage  : At line:1 char:1
                               + [System.Collections.Generic.HashSet[String]]::new(@('a', 'b', 'c', 'A …
                               + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
            CommandOrigin    : Internal
        ScriptStackTrace      : at <ScriptBlock>, <No file>: line 1
    TargetSite  : System.Object CallSite.Target(System.Runtime.CompilerServices.Closure, System.Runtime.CompilerServices.CallSite, System.Type, System.Object[], System.StringComparer)
    Message     : Cannot find an overload for "new" and the argument count: "2".
    Data        : System.Collections.ListDictionaryInternal
    Source      : Anonymously Hosted DynamicMethods Assembly
    HResult     : -2146233087
    StackTrace  :
   at CallSite.Target(Closure , CallSite , Type , Object[] , StringComparer )
   at System.Dynamic.UpdateDelegates.UpdateAndExecute3[T0,T1,T2,TRet](CallSite site, T0 arg0, T1 arg1, T2 arg2)
   at System.Management.Automation.Interpreter.DynamicInstruction`4.Run(InterpretedFrame frame)
   at System.Management.Automation.Interpreter.EnterTryCatchFinallyInstruction.Run(InterpretedFrame frame)
CategoryInfo          : NotSpecified: (:) [], MethodException
FullyQualifiedErrorId : MethodCountCouldNotFindBest
InvocationInfo        :
    ScriptLineNumber : 1
    OffsetInLine     : 1
    HistoryId        : -1
    Line             : [System.Collections.Generic.HashSet[String]]::new(@('a', 'b', 'c', 'A', 'b'), [StringComparer]::InvariantCultureIgnoreCase)
    PositionMessage  : At line:1 char:1
                       + [System.Collections.Generic.HashSet[String]]::new(@('a', 'b', 'c', 'A …
                       + ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    CommandOrigin    : Internal
ScriptStackTrace      : at <ScriptBlock>, <No file>: line 1

Environment data

Name                           Value
----                           -----
PSVersion                      7.2.0
PSEdition                      Core
GitCommitId                    7.2.0
OS                             Microsoft Windows 10.0.18363
Platform                       Win32NT
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0…}
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1
WSManStackVersion              3.0

Workarround

To workaround this you will need specify the array item type ([string[]]):

[System.Collections.Generic.HashSet[String]]::new([String[]]@('a', 'b', 'c', 'A', 'b'), [StringComparer]::InvariantCultureIgnoreCase)

Related

This is probably related to the fact that I can't directly cast an object[] to a specific enumerable either:

[System.Collections.Generic.IEnumerable[String]]@('a', 'b', 'c')
InvalidArgument: Cannot convert the "System.Object[]" value of type "System.Object[]" to type "System.Collections.Generic.IEnumerable`1[System.String]".

But this (rather WET) statement works:

[System.Collections.Generic.IEnumerable[String]][String[]]@('a', 'b', 'c')
a
b
c

Related issues

• #16002 IEnumerable -property values should enumerate
• #16003 Accelerate HashSet

[SeeminglyScience]

Also related: #7651

[SeeminglyScience]

The Engine-WG discussed this last night and we agree that casting should be aware of a few key interfaces and how to translate them.

The big ones I can see being necessary for a first round is:

Interface	Fallback
IList<T>	T[]
ICollection<T>	T[]
IEnumerable<T>	T[]
IReadOnlyList<T>	T[]
IReadOnlyCollection<T>	T[]
Less important
IList	object[]
ICollection	object[]
IEnumerable	object[]
IDictionary	Hashtable
IDictionary<TKey, TValue>	Dictionary<TKey, TValue>

Marking up for grabs.

[iRon7]

This is already labeled Up-for-Graps and therefore shouldn't be closed.

[iRon7]

The issue still exists in PowerShell 7.4.2 and is already labeled Up-for-Graps and therefore shouldn't be closed. Or???