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ASP.NET: Dependency Injection

Dependency

When one object (Object A) references another object (Object B), using its properties and methods, it means that the first object (A) depends on the second (B), making the second object (B) a dependency.

// Object A - the class that depends on Object B class Reviewer { private EmailSender Sender = new EmailSender(); public SendReview(string lesson, string comments) { Sender.SendReview(lesson, comments); } } // Object B - the dependency class EmailSender { public SendReview(string lesson, string comments) { // Send an email with the Lesson and Comments } } static void Main(string[] args) { Reviewer reviewer = new Reviewer(). SendReview("Dependecy Injection", "Learned a ton!"); }

Dependency Injection

When a service (dependency object) is created outside of its client (the object that depends on it) that service can be passed into, or injected, into the client, typically through the client’s constructor.

This process is called dependency injection, where services are not created by the clients that use (and depend on) them, but rather are created and managed in other code, and are injected into the client.

//The client class that depends on a service class Reviewer { public EmailSender _sender {get; set;} //EmailSender is injected into the Reviewer object public Reviewer(EmailSender sender) { _sender = sender; } public SendReview(string lesson, string comments) { _sender.SendReview(lesson, comments); } } //The dependency/service class class EmailSender { public SendReview(string lesson, string comments) { // Send an email with the Lesson and Comments } } static void Main(string[] args) { EmailSender sender = new EmailSender(); //Injecting Sender into Reviewer Reviewer reviewer = new Reviewer(Sender). SendReview("Dependecy Injection", "Super helpful!"); }

IoC Container

The IoC Container (Inversion of Control Container) is a framework that acts as the dependency injector. This allows the programmer to focus on using the service within the classes that depend on it, rather than managing the entire life cycle of the service.

The IoC Container does all of the following:

  1. Registers services with a concrete implementation (a class)
  2. Instantiating, or resolving, the service class to be injected into the client class
  3. Injecting the service
  4. Disposing of the service instance based on the registered settings

Registering Services

Services are registered in the ConfigureServices() method of the Startup class in Startup.cs.

Once the services are registered, they are available for injection into client classes that use those services.

Services are registered using the AddTransient(), AddScoped(), and AddSingleton() methods. These methods dictate how the service’s life cycle is managed.

  • AddTransient - the service is created each time it’s requested from the IoC Container. This means that when more than one class uses the service, those classes will be injected with a fresh new instance of that service, even if it’s within the same request.

  • AddScoped - the service is created for each client request. If multiple classes use and are injected with the service within the same request, only one instance of that service is created and used throughout the request for those classes.

  • AddSingleton - the service is created once on the first time the service is requested and is instantiated for the lifetime of the application process.

public void ConfigureServices(IServiceCollection services) { services.AddRazorPages(); services.AddTransient<ITransientService, TransientService>(); services.AddScoped<IScopedService, ScopedService>(); services.AddSingleton<ISingletonService, SingletonService>(); }

AddRazorPages()

Following the services.Add{ServiceName} naming convention, the AddRazorPages() method registers all the services required for the web app to function as a Razor Pages application.

AddRazorPages() is called within Startup.ConfigureServices().

If you’re curious and want to peek under the hood to see all the services that are registered within AddRazorPages(), you can find the code at dotnet/aspnetcore. Remember, it’s all open source!

public void ConfigureServices(IServiceCollection services) { services.AddRazorPages(); }

AddDbContext

The AddDbContext<T>() method registers the framework-provided services that allow all page models to be injected with an instance of the T database context that will be used to access the application’s database.

When calling the AddDbContext<T>() method to register the application’s database context service, one must also pass in an instance of the DbContextOptions object that contains information such as the database provider type (UseSqlServer(), UseSqlite(), etc.), connection string (defined in appsettings.json), and other optional settings that defines the behavior of the context.

The AddDbContext<T>() method is called within Startup.ConfigureServices().

public void ConfigureServices(IServiceCollection services) { services.AddDbContext<MyAppContext>(options => options.UseSqlServer( Configuration.GetConnectionString("MyAppContext"))); }

Dependencies As Interfaces

In order to satisfy the Dependency Inversion Principle, injected services are typically referenced as interfaces. This allows the client class to use an implemented service whose behavior is well defined via its interface, and not have any knowledge or be concerned with the actual concrete class that implements that interface.

Any changes to the concrete class’s methods or properties would not require any modifications to the client class since it only knows of the interface and its well defined abstract methods.

public class ReviewModel : PageModel { // Notice IFormSender interface private readonly IFormSender _Sender; [BindProperty] public string Review {get;set;} [BindProperty] public int ProductID {get;set} // Notice IFormSender interface public ReviewModel(IFormSender sender) { _Sender = sender; } public async Task<IActionResult> OnPost() { await _Sender.SubmitReview(ProductID, Review); return RedirectToPage("/Index"); } }

DI and IOC Container

A built-in IoC Container (Inversion of Control Container) is provided with ASP.NET that implements all dependency injection functionality and allows the developer to implement structured code following the Dependency Inversion (DIP) and SOLID principles.

The IoC Container allows the developer to register services for injection in the Startup.ConfigureServices() method.

The IoC Container handles the lifecycles of services and lets the developer implement classes that use the registered services to perform work.

//Registering the Service public class Startup { public void ConfigureServices(IServiceCollection services) { services.AddRazorPages(); services.AddScoped<ISendService, SendService>(); } } //Injecting the service public class ReviewModel : PageModel { private readonly ISendService _sender; public ReviewModel(ISendService sender) { _sender = sender; } }

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