fix: Instantiate() behavior for Prefab and Entity references

sources/engine/Stride.Engine.Tests/TestEntity.cs

@@ -176,7 +176,13 @@ public void TestCloningBehavior()
         {
             var externalEntity = new Entity();
             var sourceEntity = new Entity();
-            var sourceComponent = new EntityComponentWithPrefab { Prefab = new Prefab(), ExternalEntityRef = externalEntity/*, ExternalComponentRef = externalEntity.Transform*/ };
+            var sourceComponent = new EntityComponentWithPrefab
+            {
+                Prefab = new Prefab(),
+                ExternalEntityRef = externalEntity,
+                // Not yet supported, see commented out ExternalComponentRef declaration further and PR #2914
+                //ExternalComponentRef = externalEntity.Transform
+            };
             sourceComponent.Prefab.Entities.Add(sourceEntity);
             sourceEntity.Add(sourceComponent);
 
@@ -192,7 +198,7 @@ public void TestCloningBehavior()
             Assert.Equal(clonedComponent.ExternalEntityRef, sourceComponent.ExternalEntityRef);
 
             // References to entity component outside this one's hierarchy should not clone the component referenced, it should point to the same reference
-            // Not yet supported
+            // Not yet supported, see commented out ExternalComponentRef declaration further and PR #2914
             /*Assert.Equal(clonedComponent.ExternalComponentRef, sourceComponent.ExternalComponentRef);*/
         }
 
@@ -295,6 +301,14 @@ public class EntityComponentWithPrefab : EntityComponent
     {
         public required Prefab Prefab { get; set; }
         public required Entity ExternalEntityRef { get; set; }
+
+        /* TODO:
+         * References to entity component outside of a prefab's hierarchy should not clone the component referenced, it should point to the same reference.
+         * More work is required on that front, particularly with EntityComponent which does not have a specific serializer,
+         * we would need one that derive from DataContentSerializerWithReuse to properly filter external references.
+         * The serializer for TransformComponent derives from a simple DataSerializer for example.
+         * See also PR #2914
+         */
         /*public required TransformComponent ExternalComponentRef { get; set; }*/
     }
 }

sources/engine/Stride.Engine/Engine/Design/EntityCloner.cs

@@ -48,12 +48,12 @@ private static HashSet<object> ClonedObjects()
         }
 
         /// <summary>
-        /// Clones the specified prefab.
-        /// <see cref="Entity"/>, children <see cref="Entity"/> and their <see cref="EntityComponent"/> will be cloned.
+        /// Instantiate the content of the prefab provided.
+        /// <see cref="Prefab.Entities"/>, children <see cref="Entity"/> and their <see cref="EntityComponent"/> will be cloned.
         /// Other assets will be shared.
         /// </summary>
-        /// <param name="prefab">The prefab to clone.</param>
-        /// <returns>A cloned prefab</returns>
+        /// <param name="prefab">The prefab to instantiate.</param>
+        /// <returns>A clone of this prefab's <see cref="Prefab.Entities"/></returns>
         public static List<Entity> Instantiate(Prefab prefab)
         {
             if (prefab == null) throw new ArgumentNullException(nameof(prefab));
@@ -131,15 +131,9 @@ internal static void CollectEntityTreeHelper(Entity entity, HashSet<object> enti
         /// <returns>The cloned object.</returns>
         private static T Clone<T>(HashSet<object> clonedObjects, TryGetValueFunction<object, object> mappedObjects, T entity) where T : class
         {
-            if (cloneSerializerSelector == null)
-            {
-                cloneSerializerSelector = new SerializerSelector(true, false, "Default", "Clone");
-            }
+            cloneSerializerSelector ??= new SerializerSelector(true, false, "Default", "Clone");
 
-            if (entitySerializerSelector == null)
-            {
-                entitySerializerSelector = new SerializerSelector(true, false, "Default");
-            }
+            entitySerializerSelector ??= new SerializerSelector(true, false, "Default");
 
             // Initialize CloneContext
             lock (cloneContext)