Hello dear friends. In this video, we will start to examine SOLID in design patterns. So, let's begin. The SOLID principles were first conceptualized by Robert C. Martin in his 2000 paper, "Design Principles and Design Patterns". These concepts were later built upon by Michael Feathers, who introduced us to the SOLID acronym. And in the last 20 years, these five principles have revolutionized the world of object-oriented programming changing the way that we write software. So, what is SOLID, and how does it help us write better code?

Simply put, Martins and Feathers Design Principles encourage us to create a more maintainable, understandable and flexible software. Consequently, as our applications grow in size, we can reduce their complexity and save ourselves a lot of headaches further down the road. Once written, code cannot stay the same forever. In fact, the reason of SOLID comes up is hidden in this sentence. For instance, three years ago we completed a sizable project and delivered it to the client. The customer requested that we add new features to the project three years later. Consequently, updating a project that was built on sound principles is simple and quick. 

So, we can say that the main purpose of SOLID principles is to allow developers to write code that can keep pace with these changes with minimum effort rather than better code writing. So, what are these SOLID principles? S, Single Responsibility. O, Open/Closed. L, Liskov Substitution. I, Interface Segregation. D, Dependency Inversion. While some of these words may sound daunting, they can be easily understood with some simple code examples. In the following sections, we will explain them one by one. Next video, we'll examine Single Responsibility. See you in the next video.