Server-side validation vs client-side validation (unobtrusive)?

Short answer: Server-side: Always performed on the server; use ModelState.IsValid. Client-side: HTML5 + jQuery Unobtrusive Validation (for MVC/Razor Pages). Client-side validation improves UX, but server-side is essential for security.

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.