What is Dependency Injection in ASP.NET Core?

Dependency Injection is a design pattern used to implement IoC, allowing for loosely coupled code and easier testing.

What are the DI lifetimes in ASP.NET Core?

Transient, Scoped, and Singleton are the 3 DI lifetimes available in ASP.NET Core.

Where is DI configured in an ASP.NET Core app?

It is configured in the Program.cs file using builder.Services.

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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