Several implementations of one interface are registered in ASP.NET Core DI. What happens when you resolve one service versus IEnumerable, and when would keyed services or a factory help?

I avoid using service location as a general branching mechanism because it hides dependencies and moves missing registrations to runtime. Adding a registration can be a behavior change when an enumerable pipeline now runs another component or when single resolution picks a different last item, so ordering and duplicate rules need tests. A singleton factory cannot safely retain scoped strategy instances; it either remains scoped or resolves inside an owned scope. For plugin-like extensibility I validate keys, duplicates, and required capabilities at startup. In production I log the selected strategy name without sensitive configuration and make unknown keys fail clearly rather than falling back to an arbitrary default.