ADDomainController: Fix Test-TargetResource Issue When the 'ReadOnlyReplica' Property is set to 'true'

CHANGELOG.md

@@ -31,6 +31,9 @@ For older change log history see the [historic changelog](HISTORIC_CHANGELOG.md)
   - Restored importing the `DscResource.Common` module import in the `ActiveDirectoryDsc.Common` module that was
     incorrectly disabled.
     ([issue #612](https://github.com/dsccommunity/ActiveDirectoryDsc/issues/612)).
+- ADDomainController
+  - Fixed `Test-TargetResource` error when the `ReadOnlyReplica` property is set to `true`
+    ([issue #611](https://github.com/dsccommunity/ActiveDirectoryDsc/issues/611)).
 - ADReplicationSiteLink
   - Fixed setting options after the resource is initially created
     ([issue #605](https://github.com/dsccommunity/ActiveDirectoryDsc/issues/605)).

source/DSCResources/MSFT_ADDomainController/MSFT_ADDomainController.psm1

@@ -709,7 +709,7 @@ function Test-TargetResource
 
     if ($PSBoundParameters.ContainsKey('ReadOnlyReplica') -and $ReadOnlyReplica)
     {
-        if ($testTargetResourceReturnValue -and -not $testTargetResourceReturnValue.ReadOnlyReplica)
+        if ($testTargetResourceReturnValue -and -not $existingResource.ReadOnlyReplica)
         {
             New-InvalidOperationException -Message $script:localizedData.CannotConvertToRODC
         }

tests/Unit/MSFT_ADDomainController.Tests.ps1

@@ -370,6 +370,7 @@ try
                             AllowPasswordReplicationAccountName = @($allowedAccount)
                             DenyPasswordReplicationAccountName  = @($deniedAccount)
                             FlexibleSingleMasterOperationRole   = @('DomainNamingMaster', 'RIDMaster')
+                            ReadOnlyReplica                     = $true
                             Ensure                              = $true
                         }
                     }
@@ -452,6 +453,18 @@ try
                         Assert-MockCalled -CommandName Get-TargetResource -Exactly -Times 1
                     }
                 }
+
+                Context 'When property ReadOnlyReplica and SiteName are in desired state' {
+                    It 'Should return $true' {
+                        $result = Test-TargetResource @testDefaultParams -DomainName $correctDomainName `
+                            -ReadOnlyReplica $true -SiteName $correctSiteName
+                        $result | Should -Be $true
+                    }
+
+                    It 'Should call the expected mocks' {
+                        Assert-MockCalled -CommandName Get-TargetResource -Exactly -Times 1
+                    }
+                }
             }
 
             Context 'When the system is not in the desired state' {