Fail IPublishedContentFactory faster

src/Umbraco.Core/Models/PublishedContent/PublishedContentExtensionsForModels.cs

@@ -1,4 +1,6 @@
-namespace Umbraco.Core.Models.PublishedContent
+using System;
+
+namespace Umbraco.Core.Models.PublishedContent
 {
     /// <summary>
     /// Provides strongly typed published content models services.
@@ -12,9 +14,32 @@
         /// <returns>The strongly typed published content model.</returns>
         public static IPublishedContent CreateModel(this IPublishedContent content)
         {
-            return PublishedContentModelFactoryResolver.Current.HasValue
-                ? PublishedContentModelFactoryResolver.Current.Factory.CreateModel(content)
-                : content;
+            if (content == null)
+                return null;
+
+            if (PublishedContentModelFactoryResolver.Current.HasValue == false)
+                return content;
+
+            // get model
+            // if factory returns nothing, throw
+            // if factory just returns what it got, return
+            var model = PublishedContentModelFactoryResolver.Current.Factory.CreateModel(content);
+            if (model == null)
+                throw new Exception("IPublishedContentFactory returned null.");
+            if (ReferenceEquals(model, content))
+                return content;
+
+            // at the moment, other parts of our code assume that all models will
+            // somehow implement IPublishedContentExtended and not just be IPublishedContent,
+            // so we'd better check this here to fail as soon as we can.
+            //
+            // see also PublishedContentExtended.Extend
+
+            var extended = model as IPublishedContentExtended;
+            if (extended == null)
+                throw new Exception("IPublishedContentFactory created an object that does not implement IPublishedContentModelExtended.");
+
+            return model;
         }
     }
 }