Why does Unity custom BuilderStrategy returns null even after setting Existing property

Question

I'm using version 2 of Unity (that comes with Prism4). I'm trying to write an extension that can return enumerable of a non registered type. Following code is what I've written but I'm getting null after resolve call.

        class EnumerableStrategy : BuilderStrategy
        {
            public override void PreBuildUp(IBuilderContext context)
            {
                context.Existing = new []{"Test"};
                context.BuildComplete = true;
            }
        }

        class EnumerableExtension : UnityContainerExtension
        {
            protected override void Initialize()
            {
                Context.BuildPlanStrategies.AddNew<EnumerableStrategy>( Microsoft.Practices.Unity.ObjectBuilder.UnityBuildStage.PreCreation);
            }
        }
        static void Main(string[] args)
        {

            var container = new UnityContainer();
            container.AddNewExtension<EnumerableExtension>();
            var items = container.Resolve<IEnumerable<string>>();
            foreach (var item in items)
                Console.WriteLine(item.ToString());
        }

Items variable turns out to be null. Why?

Answer 1

Add the strategy to the Strategies collection, not the BuildPlanStrategies.

BuildPlanStrategies is for constructing the objects that will construct the resolved objects. That's not what you're doing - you're just returning the objects directly. I'm surprised you got null, actually - I would have expected an invalid cast exception in there somewhere.