Select-Object and New-MockObject - Discarded PSObject

[chrisdent-de]

Checklist

• Issue has a meaningful title
• I have searched the existing issues. See all issues
• I have tested using the latest version of Pester. See Installation and update guide.

What is the issue?

When writing unit tests, and mocking complex objects, I want to make use of New-MockObject.

New-MockObject wires up members to the PSObject wrapper, hiding the real properties (or methods). This is a good thing.

New-MockObject avoids Add-Member to achieve this, likely because Add-Member is not regarded as being particularly fast.

This has an unfortunate side-effect if Select-Object is being used in the test subject's code. Select-Object replaces the PSObject wrapper on the selected object, discarding NoteProperty members added via PSObject.Properties.

https://github.com/PowerShell/PowerShell/blob/master/src/Microsoft.PowerShell.Commands.Utility/commands/utility/Select-Object.cs#L573

Expected Behavior

The following tests describe the expected behaviour. All tests should pass:

$container = New-PesterContainer -ScriptBlock {
    Context 'When using PSObject.Properties.Add' {
        BeforeAll {
            $mockObject = New-MockObject -Type object -Properties @{
                Nested = New-MockObject object -Properties @{
                    Name = 'Value'
                }
            }    
        }

        It 'allows access to the property using the member dereference operator' {
            $mockObject.Nested.Name | Should -Be 'Value'
        }

        It 'allows access to the property using Select-Object' {
            # This test currently fails.
            $mockObject |
                Select-Object -ExpandProperty Nested |
                Select-Object -ExpandProperty Name |
                Should -Be 'Value'
        }
    }

    Context 'When using Add-Member' {
        BeforeAll {
            $addMember = New-MockObject -Type object | Add-Member -PassThru -NotePropertyName 'Nested' -NotePropertyValue (
                New-MockObject object | Add-Member -PassThru -NotePropertyName 'Name' -NotePropertyValue 'Value'
            )
        }

        It 'allows access to the property using the member dereference operator' {
            $addMember.Nested.Name | Should -Be 'Value'
        }

        It 'allows access to the property using Select-Object' {
            # This test currently fails.
            $addMember |
                Select-Object -ExpandProperty Nested |
                Select-Object -ExpandProperty Name |
                Should -Be 'Value'
        }
    }
}
Invoke-Pester -Container $container -Output Detailed

Steps To Reproduce

This problem is exhibited by any object accessed by Select-Object where members have been added using .PSObject.Properties.

$object = [object]::new()
$object.PSObject.Properties.Add(
    [PSNoteProperty]::new(
        'Nested',
        [object]::new()
    )
)
$object.Nested.PSObject.Properties.Add(
    [PSNoteProperty]::new(
        'Name',
        'Value'
    )
)
# This works
$object.Nested.Name
# this doesn't
$object | Select-Object -ExpandProperty Nested | Select-Object -ExpandProperty Name

It is an unfortunate side effect of this problem which affects Pester. However, Pester is the far easier "thing to fix" than PowerShell, so Add-Member may be used to work-around this problem.

Describe your environment

Pester version     : 5.4.0 C:\Development\_modules\Pester\5.4.0\Pester.psm1
PowerShell version : 7.3.6
OS version         : Microsoft Windows NT 10.0.19045.0

Possible Solution?

No response

[fflaten]

Thanks for the detailed report and troubleshooting. Will need to compare and benchmark later, but initial thought is that we could probably use Add-Member in New-MockObject specifically. As it's an optional helper, the performance savings of the current .NET approach are not worth the trouble. Hopefully there's no major side effects of changing.

[nohwnd]

We use the property access directly for performance, if that has bugs that we cannot fix, I am okay with reverting back to Add-Member under New-MockObject. @chrisdent-de Please consider PRing this. Please use the add-member from safeCommands table.

[fflaten]

Yeah this is really broken. Tried switching internals to Add-Member -Force but still fails. We hit this problem where even Add-Member fails if you add members post variable assignment for some reason.

$nestedObject = [object]::new()
$nestedObject | Add-Member -NotePropertyName Name -NotePropertyValue 'Value'
$object = [object]::new()
$object | Add-Member -NotePropertyName Nested -NotePropertyValue $nestedObject
# This works
$object.Nested.Name
# this fails
$object | Select-Object -ExpandProperty Nested | Select-Object -ExpandProperty Name

# Replacing the start with this works:
$nestedObject = [object]::new() | Add-Member -NotePropertyName Name -NotePropertyValue 'Value' -PassThru