What is Dependency Injection?

[MUSIC] 0:00 Hi, my name is James. 0:04 In this workshop, you'll learn about dependency injection and 0:06 how to use it in an ASP.NET MVC application. 0:11 We'll start with an overview of what dependencies are, and 0:15 how dependency injection, or DI, 0:17 can help you to improve the manageability and testability of your code. 0:20 We'll also see how to select, install, and 0:25 configure one of the popular DI containers available for ASP.NET MVC. 0:28 And we'll finish up with a look at DI in the ASP.NET Core web framework. 0:33 We won't be covering in this workshop how to use 0:39 DI with the ASP.NET Web API framework. 0:42 That'll be covered in a future course on Web API, see the teacher's notes for 0:45 more information. 0:49 Here's the project that we'll be using in this workshop, 0:51 open in the community edition of Visual Studio 2017. 0:54 Our project name is Treehouse.SolarSystem. 0:56 If you'd like to follow along, see the teacher's notes for 1:01 a link to download the project files. 1:03 Our application is a very basic ASP.NET MVC application. 1:05 We have a single controller named PlanetsController, 1:10 and we have a single model named Planet. 1:15 We also have some entity framework related classes in the Data folder. 1:19 We have a Context class, a Database Initializer class, 1:24 that seeds our database with some data, and a single Repository class. 1:29 Let's see what our application does. 1:34 And as you might expect, it displays a list of the planets in our solar system, 1:37 all nine, I mean eight of them, sorry Pluto. 1:41 Let's review how our application displays the list of planets that you just saw. 1:45 Here's the index action method that displays the list of planets. 1:49 We can see here that it's calling a method called GetPlanets 1:53 on a repository private field. 1:57 The repository private field is of type Repository, which 1:59 you can see in our class constructor, we're instantiating an instance. 2:04 We're also instantiating an instance of the Context class, and 2:08 passing that into the repository's constructor. 2:11 The Repository class, GetPlanets method, is using the Context class to query 2:17 the underlying database for the list of planets. 2:22 Back in the controller, we can see that we're also overriding the Dispose method, 2:26 which is being used to dispose the _context 2:31 when our controller is being disposed. 2:34 We're doing this in order to control the lifetime of our _context object. 2:36 So, what are dependencies? 2:41 As we just saw, our controller class, well specifically the index action method, 2:43 is dependent upon an instance of the repository class. 2:48 And, the repository class is dependent upon an instance of the context class. 2:52 This chain of dependencies is sometimes called a dependency tree or 2:59 a dependency graph. 3:03 The context and repository classes are examples of database or data dependencies. 3:05 There are many other types of dependencies that your application might have. 3:10 External dependencies, like storage, messaging services, 3:15 like email or texting, or any other restful service, or HTTP API. 3:19 Or maybe internal dependencies like app settings, user authentication, 3:24 session management or really any business logic that's specific to your application, 3:29 for instance, maybe a shopping card for an e-commerce application. 3:35 Our controller shouldn't really be concerned with instantiating instances 3:40 of its dependencies, or controlling the lifetime of those dependencies. 3:44 Ideally, we should just be able to ask for our dependencies. 3:49 One way to do that would be to declare our dependencies 3:53 through constructor parameters. 3:56 Let's take a look at what that would look like. 3:58 Let's add a parameter of type Repository named repository. 4:01 Then, we can replace this instantiation of the repository class with a reference to 4:08 that parameter. 4:13 We can also get rid of this instantiation of the context class. 4:16 And, this boolean disposed private field and the context private field. 4:21 Even more, we can get rid of this dispose override. 4:28 That's a lot less code, isn't it? 4:36 With this approach, the underlying app framework would provide our controller 4:39 with an instance of the repository class. 4:43 And for our repository class, it would supply an instance of the context class. 4:48 And, it would also manage the live time of the context for us. 4:54 This technique is known as constructor injection. 4:58 Constructor injection is part of a broader design pattern 5:01 known as dependency injection. 5:04 Dependency injection, or 5:07 DI, is related to the inversion of control, or IOC design principle. 5:08 As we just saw in our example, using DI classes, like our controller, 5:14 no longer need to concern themselves with how their dependencies are instantiated. 5:19 In fact, any of our controller's dependencies, like the repository class, 5:23 in turn might have their own dependencies. 5:27 That's no problem. 5:30 It's still not the concern of our controller. 5:31 Directly instantiating dependencies within a class, 5:34 tightly couples that class to its dependencies. 5:37 Using DI helps us to create loosely coupled classes or components. 5:40 Loose coupling improves code reusability, 5:44 making it easier to reuse our classes in new ways or in new environments. 5:47 DI also helps to improve the testability of our code. 5:53 For example, if we wanted to unit test our controller's index action method, 5:56 we could provide the controller with an instance of the repository class 6:01 that would avoid querying the underlying database. 6:04 Instead, it'd simply return a static collection of planets. 6:07 Supplying this mock repository class to the controller class 6:11 would allow our unit test to run in a performant predictable way. 6:16 Constructor injection isn't the only way to implement DI. 6:20 Property injection and 6:23 method injection are two alternative approaches that you can use. 6:25 We won't be covering these approaches in this workshop, 6:28 as we'll be using constructor injection. 6:31 For more information about the IOC design principle, property injection, 6:34 method injection, and an example of DI and unit testing, see the teacher's notes. 6:38 Dependency injection is a popular design pattern. 6:44 DI can be used with virtually any language or platform, with or 6:47 without the support of an external library. 6:50 Google's Angular framework and Microsoft's ASP.NET Core web framework, 6:54 as we'll see later in this workshop, provide built-in support for DI. 6:58 Unfortunately, the ASP.NET MVC web framework doesn't provide support for 7:02 DI out of the box. 7:08 You need to install and configure what's known as a DI container. 7:09 In the next video, we'll learn about DI containers and 7:13 select a DI container library to use in our project. 7:16