Dependency Inversion

Dependency Inversion, the 'D' in SOLID, is a decoupling technique where both high-level and low-level classes depend on abstractions rather than concrete implementations. Depending on concrete classes creates hard source code dependencies that make code volatile, difficult to change, and hard to test. By depending on interfaces or abstract classes instead, you can swap implementations freely, mock dependencies in tests, and extend behavior without breaking existing code. Uncle Bob's rules include: never refer to concrete classes directly, never derive from volatile concrete classes, and never override concrete functions. The principle scales from class-level design up to architectural layers in Clean Architecture and Domain-Driven Design.