Dependency Inversion Principles (DIP)

To summarize the term dependency briefly, we can explain that a class is dependent on another class, that is, a class needs another class to work. As we see in the figure below, the upper-level layer uses the lower-level layer (classes, interfaces, etc.). we can say that the level is dependent on the layer. As for the term reusability; Firstly, we think that a class can be written and used in various parts of the project, but we can define a code that we use as a real reuse and a higher level library can be used in other projects without touching the code. We cannot do this because of the interdependence of classes in well-designed software. At this point, Dependency Inversion Principle (DIP), which is an important software principle that needs to be implemented in order to develop more flexible and reusable modules, comes into play. Briefly explain this method;

Upper-level modules, classes, etc., lower-level classes must not be dependent on modules. Lower-level modules must be dependent on higher-level modules (interfaces of modules). For short, we call it Dependency Inversion.

Let's see how I applied this principle in the next example:

On the left side we found an Layered Application where the BusinessLogic depends on the SqlDatabase implementation. It is a coupled way to write code. On the right side, by adding an IRepository and applying DIP then the SqlDatabase has its dependency pointing inwards.