SOLID Principles Explained: Five Rules for Code That Withstands Change
A deep dive into SOLID with real C# violations and fixes for each principle, going beyond definitions to the actual code smells behind them.
This piece revisits SOLID, a staple of senior engineering interviews, by going beyond dictionary-style definitions. For each principle it pairs a plain-English explanation, a real-world analogy, and a concrete C# example showing both the violation and the corrected version, focusing on the actual code smell each principle targets rather than just its name.
For Single Responsibility, it corrects the common misconception that a class must do only one thing, clarifying that the real test is whether a single business change maps to a single reason to modify the class. For Open/Closed, it shows how a growing switch statement is replaced with polymorphic classes, so each new case becomes a new class instead of a risky edit to tested code. For Liskov Substitution, the classic Rectangle-Square example illustrates why a subclass must preserve the base type's behavioral contract, not just its structural inheritance.
The practical framing helps engineers treat SOLID not as abstract theory but as a diagnostic tool for spotting maintenance risks in real codebases, making it useful both for interview prep and for refactoring existing systems.