This was the start of my journey in C#; it was delightful to code with standard design patterns and make scalable software.  

Design patterns are solutions to software design problems you find again and again in real-world application development. Patterns are about reusable designs and interactions of objects.

Before diving into this article, it is highly recommended to look at Unified Modeling Language cheat sheet.

These are a compilation of publicly shared files of Do-Factory Gang Of Four, O'reilly Head First Design Patterns, Refactoring-Guru dive Into design patterns, and dive Into refactoring books with C# example projects.

This article is a concise image of application layers and four golden rules for keeping design patterns in mind while developing codes in general.