Working with JSON options: System.Text.Json vs Newtonsoft.Json?

Short answer: System.Text.Json: Default in .NET Core 3+, fast, built-in, less feature-rich. Newtonsoft.Json: More mature, supports advanced scenarios (e.g., polymorphic deserialization). Can switch to Newtonsoft by adding AddNewtonsoftJson() in MVC options.

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.