How do you implement Feature Flags dynamically via DI?

Feature flags can dictate which implementation of an interface is injected into a consumer, allowing safe A/B testing or rollouts.

A proxy class or a factory is registered in the DI container. When the interface is requested, the factory evaluates the feature flag (from a database or config) and resolves either the LegacyService or the NewService. Because it implements the same interface, the consumer is unaware.