Fix read-only properties on get operation WMI adapter

adapters/wmi/Tests/wmi.tests.ps1

@@ -108,4 +108,16 @@ Describe 'WMI adapter resource tests' {
         $res.afterState.VariableValue | Should -Be 'update'
         $res.afterState.UserName | Should -Be ("{0}\{1}" -f $env:USERDOMAIN, $env:USERNAME)
     }
+
+    It 'Get works with read-only properties on Win32_ComputerSystem' -Skip:(!$IsWindows) {
+        $manufacturer = (Get-CimInstance -ClassName Win32_ComputerSystem).Manufacturer
+        $i = @{
+            Manufacturer = $manufacturer
+        } | ConvertTo-Json
+
+        $r = dsc resource get -r root.cimv2/Win32_ComputerSystem -i $i
+        $LASTEXITCODE | Should -Be 0 
+        $res = $r | ConvertFrom-Json
+        $res.actualState.Manufacturer | Should -Not -BeNullOrEmpty
+    }
 }

adapters/wmi/wmiAdapter.psm1

@@ -122,11 +122,12 @@ function BuildWmiQuery {
         [Parameter(Mandatory = $true)]
         [string]$ClassName,
 
-        [Parameter(Mandatory = $true)]
-        [array]$Properties,
-
         [Parameter(Mandatory = $true)]
         [psobject]$DesiredStateProperties,
+
+        [Parameter()]
+        [AllowNull()]
+        [array]$Properties,
 
         [Parameter()]
         [switch]$KeyPropertiesOnly
@@ -205,6 +206,8 @@ function GetWmiInstance {
                     }
                 }
             }
+        } else {
+            $wmi_instances = Get-CimInstance -Namespace $wmi_namespace -ClassName $wmi_classname -ErrorAction Ignore -ErrorVariable Err
         }
     } else {
         $wmi_instances = Get-CimInstance -Namespace $wmi_namespace -ClassName $wmi_classname -ErrorAction Ignore -ErrorVariable Err
@@ -365,4 +368,4 @@ class dscResourceObject {
     [string] $name
     [string] $type
     [PSCustomObject] $properties
-}
+}