Decorator is not applied when component is registered by interface

[horse315]

Hi!

I'm trying to decorate a delegate component, created with a factory method, which returns an interface. I've discovered that in this case delegate is not applied.
Is it a bug or a feature?

Autofac 4.9.2

    /// <summary>
    /// package add package Autofac --version 4.9.2
    /// </summary>
    public class DecoratorTest
    {
        public interface IWork { }

        public class Work : IWork { }

        public class WorkDecorator : IWork { }

        [Fact] // fails
        public void RegisterInterface()
        {
            var containerBuilder = new ContainerBuilder();

            // interface
            containerBuilder.Register(c => (IWork)new Work()) // <== or Register<IWork>(c => new Work())
                .As<IWork>()
                .InstancePerDependency();

            // decorator
            containerBuilder.RegisterDecorator<IWork>((c, p, d) => new WorkDecorator());

            var container = containerBuilder.Build();

            var work = container.Resolve<IWork>();

            Assert.IsType<WorkDecorator>(work);
        }

        [Fact] // passes
        public void RegisterClass()
        {
            var containerBuilder = new ContainerBuilder();

            // instance
            containerBuilder.Register(c => new Work()).As<IWork>()
                .InstancePerDependency();

            // decorator
            containerBuilder.RegisterDecorator<IWork>((c, p, d) => new WorkDecorator());

            var container = containerBuilder.Build();

            var work = container.Resolve<IWork>();

            Assert.IsType<WorkDecorator>(work);
        }
    }

[tillig]

Before we take the time to try and set this up, repro it, debug it, etc... the only difference I see in these two things is in the first one, you're registering a lambda and casting it inside the lambda...

Register(c => (IWork)new Work())

...while the second one there's no cast.

Register(c => new Work())

In both cases they're registered As<IWork> and are InstancePerDependency. The decorators are also registered the same.

Am I reading that right?

[horse315]

@tillig , yes

[tillig]

Looks like we have a bug! Thanks for the report. I'm able to reproduce this. Not sure how to fix it right now, but baby steps.

[tillig]

Added failing (but skipped, for now) unit test here.

[RaymondHuy]

@tillig I think that the reason is inside this condition:
https://github.com/autofac/Autofac/blob/develop/src/Autofac/Features/Decorators/InstanceDecorator.cs#L44
This condition "registration.Activator.LimitType != instanceType" may cause bug if we have multiple types returned inside lamda expression when we register component. For example:

builder.Register(ctx => {
if (condition)
return (IDecoratedService)new ImplementorA();
else return new ImplementorA();
})

The LimitType always be the most specific type returned in the expression (in this case is IDecoratedService) but the instance type can only be determined at runtime. Therefore it results to that all registered decorators will be ignored. My solution is to remove this condition. I have re-run all test cases after removing it and they all passed. What do you think about it or do you have any better solutions :) ?

[alexmg]

The IsAdapting check seems to now be covering all the required cases. I have removed this and can committed the change. It will be included in the next release.