The Dependency Inversion Principle (DIP) is the final principle in the SOLID design principles. It states:

"High-level modules should not depend on low-level modules. Both should depend on abstractions."

"Abstractions should not depend on details; details should depend on abstractions."

These statements emphasize that software design should prioritize flexibility and decoupling by ensuring that high-level policies are not directly tied to low-level implementation details. Both high-level and low-level modules should rely on abstractions (e.g., interfaces or abstract classes) to establish a flexible relationship. This approach keeps the system modular and adaptable.

Key Concepts of DIP

• High-Level Modules: These are components that contain the core business logic of the application. They should dictate the flow of the application without being dependent on specific implementation details.
• Low-Level Modules: These modules handle specific, detailed tasks, such as database operations or network communication. They should not directly control the application's main flow.
• Abstractions: Interfaces or abstract classes that define general behaviors. High-level and low-level modules depend on these abstractions to communicate, keeping the system loosely coupled.

Understanding the Principle with an Example

Let’s say you’re developing a notification system where an EmailService sends notifications. Without DIP, the high-level module (e.g., NotificationService) would depend directly on the low-level EmailService. This makes it difficult to switch to other notification types, such as SMS or push notifications, without modifying the NotificationService.

In this design, the NotificationService is tightly coupled to EmailService, violating DIP. If we want to use a different service for notifications, we would need to modify the NotificationService class directly.

Why This Violates DIP

• Tight Coupling: The high-level module (NotificationService) is directly tied to a specific low-level implementation (EmailService). Changes in the low-level module require changes in the high-level module.
• Lack of Flexibility: Switching to another notification type (e.g., SMS or push notifications) is not possible without modifying the existing code.

In this lesson, we’ve introduced the basics of DIP and demonstrated a common violation. In the next lesson, we’ll explore how to refactor the code to follow DIP by introducing abstractions.