Added AttributeDescriptor fallback logic for DefaultTopicLookupService

This corresponds to a similar update for `DynamicTopicLookupService` (cd9e540).

Author: JeremyCaney

OnTopic/Lookup/DefaultTopicLookupService.cs

@@ -42,5 +42,30 @@ public DefaultTopicLookupService(IEnumerable<Type>? types = null, Type? defaultT
 
     }
 
+    /*==========================================================================================================================
+    | METHOD: LOOKUP
+    \-------------------------------------------------------------------------------------------------------------------------*/
+    /// <inheritdoc/>
+    /// <remarks>
+    ///   The <see cref="DefaultTopicLookupService"/> version of <see cref="Lookup(String)"/> will automatically fall back to
+    ///   <see cref="AttributeDescriptor"/> if the <paramref name="typeName"/> ends with <c>AttributeDescriptor</c>, but a
+    ///   <see cref="AttributeDescriptor"/> with the specified name cannot be found. This accounts for the fact that strongly
+    ///   typed <see cref="AttributeDescriptor"/> classes are expected to be in external plugins which are not statically
+    ///   registered with the <see cref="DefaultTopicLookupService"/>. In that case, the base <see cref="AttributeDescriptor"/>
+    ///   class will provide access to the attributes needed by most applications, including the core OnTopic library.
+    /// </remarks>
+    public override Type? Lookup(string typeName) {
+      if (typeName is null) {
+        return DefaultType;
+      }
+      else if (Contains(typeName)) {
+        return base.Lookup(typeName);
+      }
+      else if (typeName.EndsWith("AttributeDescriptor", StringComparison.OrdinalIgnoreCase)) {
+        return typeof(AttributeDescriptor);
+      }
+      return DefaultType;
+    }
+
   } //Class
 } //Namespace