Model state: checking ModelState.IsValid?

Short answer: Used to check if bound model passed validation: if (!ModelState.IsValid) {

Example code

return BadRequest(ModelState);
} MVC automatically adds errors to ModelState based on validation attributes.

Real-world example (ShopNest)

A ShopNest checkout request flows through middleware, hits a minimal API or controller, uses scoped services, and returns ProblemDetails on errors.

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.