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This chapter explores the ASP.NET Core Dependency Injection (DI) system, how to leverage it efficiently, and its limits and capabilities.We learn to compose objects using DI and delve into the Inversion of Control (IoC) principle. As we traverse the landscape of the built-in DI container, we explore its features and potential uses.Beyond practical examples, we lay down the conceptual foundation of Dependency Injection to understand its purpose, its benefits, and the problems it solves and to lay down the ground for the rest of the book as we rely heavily on DI.We then return to the first three Gang of Four (GoF) design patterns we encountered, but this time, through the lens of Dependency Injection. By refactoring these patterns using DI, we gain a more holistic understanding of how this powerful design tool influences the structure and flexibility of our software.Dependency Injection is a cornerstone in your path toward mastering modern application design and its transformative role in developing efficient, adaptable, testable, and maintainable software.In this chapter, we cover the following topics:
- What is dependency injection?
- Revisiting the Strategy pattern
- Understanding guard clauses
- Revisiting the Singleton pattern
- Understanding the Service Locator pattern
- Revisiting the Factory pattern
What is dependency injection?
DI is a way to apply the Inversion of Control (IoC) principle. IoC is a broader version of the dependency inversion principle (the D in SOLID).The idea behind DI is to move the creation of dependencies from the objects themselves to the composition root. That way, we can delegate the management of dependencies to an IoC container, which does the heavy lifting.
An IoC container and a DI container are the same thing—they’re just different words people use. I use both interchangeably in real life, but I stick to IoC container in the book because it seems more accurate than DI container.
IoC is the concept (the principle), while DI is a way of inverting the flow of control (applying IoC). For example, you apply the IoC principle (inverting the flow) by injecting dependencies at runtime (doing DI) using a container. Feel free to use any or both.
Next, we define the composition root.
The composition root
A critical concept behind DI is the composition root. The composition root is where we tell the container about our dependencies and their expected lifetime: where we compose our dependency trees. The composition root should be as close to the program’s starting point as possible, so from ASP.NET Core 6 onward, the composition root is in the Program.cs file. In the previous versions, it was in the Program or Startup classes.Next, we explore how to leverage DI to create highly adaptable systems.