What is the role of IServiceCollection and IServiceProvider in DI?

Short answer: IServiceCollection: A container used during app startup to register services and their lifetimes (Scoped, Transient, Singleton). It acts as a service registry. IServiceProvider: The built container that resolves and provides instances of registered services at runtime. It uses the registrations to create and inject dependencies.

Real-world example (ShopNest)

Patterns in ShopNest should solve a real pain (swappable payments, test seams)—not be added for decoration.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.