Lesson 4: ISP and focused interfaces

This lesson practices ISP and focused interfaces at real C# application level. We start from design pain, study code, practice a small decision, predict behavior, and verify mastery.

Prefer small role interfaces over one fat interface.

ISP and focused interfaces: Prefer small role interfaces over one fat interface.
design pressure: A repeated change, duplication, or test pain that signals the model needs a better boundary.