What is Dependency Injection and why is it used?

Short answer: Dependency Injection (DI) is a design pattern that allows you to inject dependencies (services) into classes instead of hard-coding them.

Real-world example (ShopNest)

ShopNest registers AppDbContext as Scoped and IMemoryCache as Singleton. Putting DbContext in a Singleton causes threading bugs.

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.