Lesson 3: LSP and behavioral contracts

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

A subtype must keep the promises callers rely on.

LSP and behavioral contracts: A subtype must keep the promises callers rely on.
design pressure: A repeated change, duplication, or test pain that signals the model needs a better boundary.