Mastering Dependency Injection in .NET Core: A Complete Beginner-to-Advanced Guide

Dependency Injection (DI) is a design pattern that promotes loose coupling, testability, and maintainability in .NET Core applications. The guide covers service registration in Program.cs, constructor injection, three service lifetimes (Transient, Scoped, Singleton), testing with mocked services, and advanced scenarios including factory-based registration and third-party containers like Autofac. Best practices include preferring interfaces over concrete classes, avoiding service locator patterns, and organizing registrations by feature modules.