The SOLID Design Principles Review

SOLID Principles Summary

Single Responsibility Principle (SRP)

• Concept: A class should have only one reason to change.
• Related Idea: Separation of Concerns - different classes handle independent aspects of the system.
• Implementation: Classes handle different tasks and solve different problems, which can then interact with each other.

Open/Closed Principle (OCP)

• Concept: Classes should be open for extension but closed for modification.
• Practical Meaning: Avoid modifying existing classes directly. Instead, extend their functionality using inheritance, interfaces, or dependency injection.
• Design Approach: Introduce interfaces to allow extension without altering the original class. Example: Implementing the specification pattern.

Liskov Substitution Principle (LSP)

• Concept: Subtypes must be substitutable for their base types.
• Implementation: Ensure that objects of a derived class can replace objects of the base class without altering the correctness of the program.
• Key Point: Maintain consistency in class design to avoid issues when consumers of your API use base types.

In C#, we use virtual keyword and override to implement the derived class.

Interface Segregation Principle (ISP)

• Concept: Avoid putting too much in an interface; keep them small and focused.
• Practical Meaning: Large interfaces force implementers to provide unnecessary implementations.
• Implementation: Split large interfaces into smaller, more specific ones to avoid forcing unnecessary implementations. Related to the YAGNI (You Ain’t Gonna Need It) principle.

Dependency Inversion Principle (DIP)

• Concept: High-level modules should not depend on low-level modules. Both should depend on abstractions.
• Implementation: Use interfaces to abstract low-level details. High-level modules can then depend on these abstractions rather than the concrete details.
• Example: Instead of exposing a collection directly, provide query mechanisms through an interface like IEnumerable or IQueryable. High-level modules can interact with this interface without knowing the underlying implementation details.

These principles form the foundation of good object-oriented design, promoting flexibility, maintainability, and scalability in software systems.