ActiveDirectoryDsc.Common: Test-DscPropertyState Fails Comparing $Null and Arrays

[X-Guardian]

Details of the scenario you tried and the problem that is occurring

The Test-DscPropertyState function in the ActiveDirectoryDsc.Common module raises an exception in the following scenarios:

• When evaluating an array, but the current value is $null
• When evaluating an array, when both values are $null

This is because $null does not have a Get-Type() method and Compare-Object can't take $null as an input.

Verbose logs showing the problem

[-] Should return false when evaluating an array, but the desired value is $null 2ms
        RuntimeException: You cannot call a method on a null-valued expression.
        at Test-DscPropertyState, \\nas01\data\users\simon\documents\GitHub\X-Guardian\ActiveDirectoryDsc\Modules\ActiveDirectoryDsc.Common\ActiveDirectoryDsc.Common.psm1: line 1806
[-] Should return true when evaluating an array, when both values are $null 3ms
        RuntimeException: You cannot call a method on a null-valued expression.
        at Test-DscPropertyState, \\nas01\data\users\simon\documents\GitHub\X-Guardian\ActiveDirectoryDsc\Modules\ActiveDirectoryDsc.Common\ActiveDirectoryDsc.Common.psm1: line 1804

Suggested solution to the issue

Add code to Test-DscPropertyState to check $Values.CurrentValue and $Values.DesiredValues against $null.

New Pester Tests Showing the Issues

It 'Should return false when evaluating an array, but the desired value is an empty array' {
    $mockValues = @{
        CurrentValue = @('1','2')
        DesiredValue = @()
    }

    Test-DscPropertyState -Values $mockValues | Should -BeFalse
}

It 'Should return true when evaluating an array, when both values are $null' {
    $mockValues = @{
        CurrentValue = $null
        DesiredValue = $null
    }

    Test-DscPropertyState -Values $mockValues -Verbose | Should -BeTrue
}

The operating system the target node is running

OsName               : Microsoft Windows 10 Pro
OsOperatingSystemSKU : 48
OsArchitecture       : 64-bit
WindowsVersion       : 1903
WindowsBuildLabEx    : 18362.1.amd64fre.19h1_release.190318-1202
OsLanguage           : en-GB
OsMuiLanguages       : {en-GB, en-US, th-TH}

Version and build of PowerShell the target node is running

PSVersion                      5.1.18362.145
PSEdition                      Desktop
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
BuildVersion                   10.0.18362.145
CLRVersion                     4.0.30319.42000
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
SerializationVersion           1.1.0.1

Version of the DSC module that was used ('dev' if using current dev branch)

dev