This document discusses dependency injection in .NET. It defines dependency injection as a set of principles and patterns that enable loosely coupled code. Some benefits of loose coupling are that it is easy to extend, test, maintain, and facilitates parallel development and late binding. Common dependency injection patterns include constructor injection, property injection, and method injection. Popular dependency injection containers for .NET include Autofac, Ninject, Unity, and Castle Windsor. The document provides an example of tightly coupled code and how to make it loosely coupled using dependency injection by adding abstractions, constructor injection, and object composition.