ADUser: Update the default value logic for CommonName (#460)

- Changes to ADUser 
  - Update the logic for setting the default value for the parameter
    `CommonName`. This is due to an how LCM handles parameters when a
    default value is derived from another parameter (issue #427).

CHANGELOG.md

@@ -90,6 +90,9 @@
   - Now it correctly tests passwords when parameter DomainName is set to
    distinguished name and parameter Credential is used ([issue #451](https://github.com/PowerShell/ActiveDirectoryDsc/issues/451)).
   - Added integration tests ([issue #359](https://github.com/PowerShell/ActiveDirectoryDsc/issues/359)).
+  - Update the logic for setting the default value for the parameter
+    `CommonName`. This is due to an how LCM handles parameters when a
+    default value is derived from another parameter ([issue #427](https://github.com/PowerShell/ActiveDirectoryDsc/issues/427)).
 - Changes to ADDomain
   - BREAKING CHANGE: Renamed the parameter `DomainAdministratorCredential`
     to `Credential` to better indicate that it is possible to impersonate

DSCResources/MSFT_ADUser/MSFT_ADUser.psm1

@@ -460,7 +460,7 @@ function Get-TargetResource
         [Parameter()]
         [ValidateNotNull()]
         [System.String]
-        $CommonName = $UserName,
+        $CommonName,
 
         [Parameter()]
         [ValidateNotNull()]
@@ -734,6 +734,15 @@ function Get-TargetResource
         $ProxyAddresses
     )
 
+    <#
+        This is a workaround to make the resource able to enter debug mode.
+        For more information see issue https://github.com/PowerShell/ActiveDirectoryDsc/issues/427.
+    #>
+    if (-not $PSBoundParameters.ContainsKey('CommonName'))
+    {
+        $CommonName = $UserName
+    }
+
     Assert-Module -ModuleName 'ActiveDirectory'
 
     try
@@ -1107,7 +1116,7 @@ function Test-TargetResource
         [Parameter()]
         [ValidateNotNull()]
         [System.String]
-        $CommonName = $UserName,
+        $CommonName,
 
         [Parameter()]
         [ValidateNotNull()]
@@ -1381,6 +1390,15 @@ function Test-TargetResource
         $ProxyAddresses
     )
 
+    <#
+        This is a workaround to make the resource able to enter debug mode.
+        For more information see issue https://github.com/PowerShell/ActiveDirectoryDsc/issues/427.
+    #>
+    if (-not $PSBoundParameters.ContainsKey('CommonName'))
+    {
+        $CommonName = $UserName
+    }
+
     Assert-Parameters @PSBoundParameters
 
     $targetResource = Get-TargetResource @PSBoundParameters
@@ -1723,7 +1741,7 @@ function Set-TargetResource
         [Parameter()]
         [ValidateNotNull()]
         [System.String]
-        $CommonName = $UserName,
+        $CommonName,
 
         [Parameter()]
         [ValidateNotNull()]
@@ -1997,6 +2015,15 @@ function Set-TargetResource
         $ProxyAddresses
     )
 
+    <#
+        This is a workaround to make the resource able to enter debug mode.
+        For more information see issue https://github.com/PowerShell/ActiveDirectoryDsc/issues/427.
+    #>
+    if (-not $PSBoundParameters.ContainsKey('CommonName'))
+    {
+        $CommonName = $UserName
+    }
+
     Assert-Parameters @PSBoundParameters
 
     $targetResource = Get-TargetResource @PSBoundParameters