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Dependency Injection in ASP.NET Core

🔍 What is Dependency Injection?

Dependency Injection (DI) is a design pattern that allows objects to receive their dependencies from an external source rather than creating them internally. ASP.NET Core has built-in support for DI, which helps in creating loosely coupled, testable, and maintainable applications.

🧠 Analogy

Imagine you're going to a hotel. Instead of carrying your own pillow, towel, or soap, the hotel provides them to you. You're dependent on those items, but you don't create or bring them — they are injected into your room by the hotel service.

⚙️ How DI Works in ASP.NET Core

  • ConfigureServices in Program.cs is used to register services.
  • You use constructor injection to inject services into controllers or other classes.
public interface IMessageService {
    string GetMessage();
}

public class HelloService : IMessageService {
    public string GetMessage() => "Hello from DI!";
}

// In Program.cs
builder.Services.AddScoped<IMessageService, HelloService>();

// In Controller
public class HomeController : Controller {
    private readonly IMessageService _service;
    public HomeController(IMessageService service) {
        _service = service;
    }
}

🎯 Interview Q&A

  • Q: What is dependency injection?
    A: It is a technique where an object receives other objects it depends on, promoting loose coupling.
  • Q: How is DI supported in ASP.NET Core?
    A: Via built-in container using builder.Services in Program.cs.
  • Q: What are the DI lifetimes available?
    A: Transient, Scoped, Singleton.
  • Q: Can we replace the built-in DI container?
    A: Yes, with third-party containers like Autofac, Ninject.
  • Q: What is constructor injection?
    A: Injecting dependencies through a class constructor.
  • Q: Where do you register services in ASP.NET Core?
    A: In the ConfigureServices or builder.Services block in Program.cs.
  • Q: What is service registration?
    A: Telling the container how to create and manage dependencies.
  • Q: Can you inject services into middleware?
    A: Yes, via constructor injection or using the request pipeline context.
  • Q: What is the benefit of DI?
    A: Improves testability, modularity, and maintainability.
  • Q: Is DI mandatory in ASP.NET Core?
    A: No, but it's highly recommended and widely used.

📝 MCQs

Q1. What is Dependency Injection (DI)?

  • A way to hide code
  • Only for testing
  • A design pattern for supplying dependencies
  • A compiler feature

Q2. Which method is used to register services in ASP.NET Core?

  • Configure()
  • Main()
  • builder.Services.Add...
  • UseServices()

Q3. What is constructor injection?

  • Using static classes
  • Passing dependencies via class constructor
  • Injecting using properties
  • None of the above

Q4. Which DI lifetime creates a new instance every time?

  • Singleton
  • Scoped
  • Transient
  • Request

Q5. Which DI lifetime is shared within a request?

  • Transient
  • Scoped
  • Singleton
  • Thread

Q6. Which DI lifetime acts like a global shared service?

  • Scoped
  • Singleton
  • Transient
  • Global

Q7. Can third-party DI containers be used in ASP.NET Core?

  • No
  • Yes
  • Only in .NET Framework
  • Only with MVC

Q8. Which file typically contains service registration?

  • Startup.cs
  • HomeController.cs
  • Program.cs
  • LaunchSettings.json

Q9. Why is DI beneficial?

  • To reduce performance
  • For loose coupling and testability
  • To avoid exceptions
  • For speed

Q10. Is DI mandatory in ASP.NET Core?

  • Yes
  • No
  • No, but recommended
  • Only with Razor

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Learn More About ASP.NET Core 🚀

What is ASP.NET Core? 👉 Explained
ASP.NET Core vs .NET Framework 👉 Explained
Role of Kestrel Server in ASP.NET Core 👉 Explained
Middleware & Request Pipeline 👉 Explained
Dependency Injection in ASP.NET Core 👉 Explained
Program.cs vs Startup.cs 👉 Explained
Configuration & appsettings.json 👉 Explained
Environment-specific settings 👉 Explained
Writing Custom Middleware 👉 Explained
Logging in ASP.NET Core 👉 Explained
Static File Middleware 👉 Explained
Routing fundamentals 👉 Explained
Model Binding & Validation 👉 Explained
Razor Pages vs MVC 👉 Explained
Tag Helpers overview 👉 Explained
Filters in MVC (Action, Authorization, Exception) 👉 Explained
Web API controllers & content negotiation 👉 Explained
Versioning ASP.NET Core Web API 👉 Explained
Entity Framework Core introduction 👉 Explained
Code-First vs Database-First in EF Core 👉 Explained
Migrations in EF Core 👉 Explained
LINQ fundamentals 👉 Explained
Async/Await and async controllers 👉 Explained
Error & Exception Handling Middleware 👉 Explained
CORS configuration & usage 👉 Explained
Authentication vs Authorization 👉 Explained
ASP.NET Core Identity basics 👉 Explained
JWT Authentication integration 👉 Explained
Caching strategies 👉 Explained
Session & State Management 👉 Explained
File Upload handling 👉 Explained
Health Checks & monitoring 👉 Explained
Hosted Services & Background Tasks 👉 Explained
Working with IWebHostEnvironment 👉 Explained
IWebHostBuilder and WebHost vs Generic Host 👉 Explained
Deployment to IIS, Kestrel, Nginx, Docker 👉 Explained
Use of HTTP.sys Server 👉 Explained
Configuration providers (JSON, env, CLI) 👉 Explained
Handling Concurrency in EF Core 👉 Explained
Model validation & custom validation 👉 Explained
Dependency Injection service lifetimes 👉 Explained
Security best practices (HTTPS, HSTS, CSP) 👉 Explained
Authorization policies & claims 👉 Explained
Rate limiting & request throttling 👉 Explained
Health & metrics integration 👉 Explained
Swagger/OpenAPI documentation 👉 Explained
Blazor fundamentals 👉 Explained
Razor Class Libraries (RCL) 👉 Explained
SignalR real-time communication 👉 Explained
Performance optimization & profiling 👉 Explained
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