Lesson 5: DIP and dependency injection

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

High-level policies depend on abstractions, not concrete infrastructure.

DIP and dependency injection: High-level policies depend on abstractions, not concrete infrastructure.
design pressure: A repeated change, duplication, or test pain that signals the model needs a better boundary.