Enable OnlyIf handling for decorators and modules

[tillig]

Problem Statement

There is not currently an easy way to conditionally register decorators or modules using the OnlyIf() style syntax. You can register types conditionally...

builder.RegisterType<X>()
       .OnlyIf(b => b.IsRegistered(new TypedService(typeof(Y))));

...but you can't do that for decorators or modules. RegisterDecorator<T, U>() is a void method. While it offers the ability to conditionally apply the decorator, if you don't want the perf hit of executing the condition function on every resolve there's no easy way to conditionally register.

ContainerBuilder.RegisterModule<T>() is similar - it has a special IModuleRegistrar that gets returned, but that only allows chained module registrations and not conditions.

This issue is based on the StackOverflow question here.

Desired Solution

It would be nice to have OnlyIf support in the cases where we don't currently have it. This may require changing RegisterDecorator<T, U>() to return a new IDecoratorRegistrar or something similar such that it can add a chained callback for OnlyIf(). For modules, it's a new extension on that IModuleRegistrar.

Alternatives You've Considered

• Make DecoratorMiddleware public: While this would potentially solve the issue in the StackOverflow question (conditional registration of a decorator), it would still mean the entire contents of RegisterDecorator would be copy/pasted into another codebase and have to be managed there. If we change the set of things in Autofac that are required to make a decorator work, the external user's code would break unexpectedly.
• ... that's about it. That wouldn't solve it for modules, either.

Additional Context

Changing RegisterDecorator<T, U> to return a type instead of void is a breaking API change (I think... pretty sure) because the method signature changes. Transitive dependencies (my code => some other code that's compiled and registers decorators => Autofac) might break because it couldn't find the void version of RegisterDecorator. I'd have to test that, but... pretty sure. If so, that'd be a 7.x release and we might want to consider a new method/overload to handle that if we don't want to release the breaking change so soon after 6.0.

[ModernRonin]

I think maybe this one here falls into the same category?
https://stackoverflow.com/questions/67022317/autofac-6-register-a-decorator-as-also-implementing-another-interface

[tillig]

@ModernRonin No, that's a different thing. While it may affect the return type or handling of RegisterDecorator<T, U>, it hasn't got anything to do with enabling OnlyIf so it'd be a different issue.

[ModernRonin]

I just saw your comment on the SO question - that's a very good point, thank you :-)

[tillig]

OnlyIf for modules will be out in 6.3.0 shortly.

[tillig]

Looks like we forgot to close this one. Fixed in #1272.