Migrated GetContentTypes() to new Refresh() method

Migrated the `TopicRepositoryBase.GetContentTypes()` method to utilize the newly introduced `ContentTypeDescriptorCollection.Refresh()` method (3e3930f). This prevents it from needing to rely on the `GetContentTypes(ContentTypeDescriptor)` overload for centralizing functionality—which, in turn, depended on an unintuitive state tracking via the `_contentTypeDescriptors` field initialization to avoid a circular loop (since each function calls into the other).

As part of this, I was also able to make the `_contentTypeDescriptors` field readonly, thus helping avoid a confusing scenario where updating the cache would change which `ContentTypeDescriptorCollection` the field referenced, thus orphaning any preexisting references.

Finally, I renamed `allowedContentTypes` to `contentTypes` to avoid confusion. (`AllowsContentTypes` is a property/attribute used to track what content types can be created under an instead of a particular content type. That concept doesn't apply here, thus the name is misleading.)

Author: JeremyCaney

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -31,7 +31,7 @@ public abstract class TopicRepositoryBase : ITopicRepository {
     /*==========================================================================================================================
     | PRIVATE VARIABLES
     \-------------------------------------------------------------------------------------------------------------------------*/
-    private ContentTypeDescriptorCollection? _contentTypeDescriptors = null;
+    private readonly ContentTypeDescriptorCollection _contentTypeDescriptors = new ContentTypeDescriptorCollection();
 
     /*==========================================================================================================================
     | EVENT HANDLERS
@@ -57,20 +57,15 @@ public virtual ContentTypeDescriptorCollection GetContentTypeDescriptors() {
       /*------------------------------------------------------------------------------------------------------------------------
       | Initialize content types
       \-----------------------------------------------------------------------------------------------------------------------*/
-      if (_contentTypeDescriptors == null) {
-
-        /*----------------------------------------------------------------------------------------------------------------------
-        | Initialize cache
-        \---------------------------------------------------------------------------------------------------------------------*/
-        _contentTypeDescriptors = new ContentTypeDescriptorCollection();
+      if (_contentTypeDescriptors.Count == 0) {
 
         /*----------------------------------------------------------------------------------------------------------------------
         | Load configuration data
         \---------------------------------------------------------------------------------------------------------------------*/
         var configuration       = (Topic?)null;
 
         try {
-          configuration = Load("Configuration");
+          configuration         = Load("Configuration");
         }
         catch (TopicNotFoundException) {
           //Swallow missing configuration, as this is an expected condition when working with a new database
@@ -79,12 +74,12 @@ public virtual ContentTypeDescriptorCollection GetContentTypeDescriptors() {
         /*----------------------------------------------------------------------------------------------------------------------
         | Load root content type
         \---------------------------------------------------------------------------------------------------------------------*/
-        var allowedContentTypes = configuration?.Children.GetTopic("ContentTypes") as ContentTypeDescriptor;
+        var contentTypes        = configuration?.Children.GetTopic("ContentTypes") as ContentTypeDescriptor;
 
         /*----------------------------------------------------------------------------------------------------------------------
         | Add available Content Types to the collection
         \---------------------------------------------------------------------------------------------------------------------*/
-        _contentTypeDescriptors = GetContentTypeDescriptors(allowedContentTypes);
+        _contentTypeDescriptors.Refresh(contentTypes);
 
       }