When is IHostedService / BackgroundService used and how DI works

Short answer: there Use IHostedService or BackgroundService for tasks that run in the background. Services are injected via constructor. Scoped services must use IServiceScopeFactory.

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.