How to Answer .NET Core Dependency Injection Questions in Interviews: Expert Tips & Insights

Dependency Injection (DI) is a core concept in software development, and when you’re interviewing for a role that requires .NET Core knowledge, understanding how to answer questions related to DI can set you apart from other candidates. In this blog, we’ll guide you through the key concepts of .NET Core Dependency Injection, common interview questions, and strategies to answer them confidently. With these insights, you’ll be able to demonstrate your understanding and stand out during your next .NET Core interview.

What is Dependency Injection in .NET Core?

Dependency Injection (DI) is a design pattern that allows you to inject dependencies into a class rather than hard-coding them inside the class. In .NET Core, the built-in DI container simplifies this process, promoting cleaner code, better testability, and improved maintainability.

In .NET Core, DI is integrated into the framework by default. It allows developers to inject services such as database connections, logging, or external APIs into different components of an application. This makes code easier to test, maintain, and extend.

Common Types of Dependency Injection in .NET Core:

• Constructor Injection: Dependencies are provided through the constructor of the class.
• Property Injection: Dependencies are injected via properties.
• Method Injection: Dependencies are passed through method parameters.

Why is Dependency Injection Important in .NET Core?

• Testability: DI makes it easier to substitute dependencies with mock implementations, simplifying unit tests.
• Decoupling: By separating the creation of objects from their usage, DI helps maintain low coupling between classes.
• Maintainability: Dependencies are injected in a standardized way, making it easier to change them as needed.

Top .NET Core DI Interview Questions:

1. What is Dependency Injection in .NET Core, and why is it important?
   Explain the concept of DI and its benefits in .NET Core applications, such as improved testability, flexibility, and maintainability.
2. How do you register services in the DI container in .NET Core?
   Discuss how services are registered in Startup.cs using methods like AddTransient, AddScoped, and AddSingleton.
3. What is the difference between AddTransient, AddScoped, and AddSingleton?
   • AddTransient: Creates a new instance every time a service is requested.
   • AddScoped: Creates an instance per HTTP request.
   • AddSingleton: Creates a single instance throughout the application’s lifetime.
4. Can you explain Constructor Injection in .NET Core?
   Explain that Constructor Injection is the most common way of injecting dependencies in .NET Core. When a class is instantiated, its dependencies are passed through the constructor.
5. How would you use Dependency Injection in a web API in .NET Core?
   Discuss how services like IServiceCollection are configured and injected into controllers or services in web API applications.
6. What are the advantages of using Dependency Injection in .NET Core over manually creating objects in classes?
   Focus on loose coupling, easier unit testing, and centralized configuration of dependencies.
7. What is the role of the IServiceProvider in DI in .NET Core?
   Discuss the IServiceProvider interface and how it’s used to resolve and manage dependencies.
8. How do you resolve a dependency manually in .NET Core?
   Explain how to use the IServiceProvider to resolve services if necessary (although it’s not recommended in most scenarios).

Best Practices for Answering DI Questions in Interviews:

• Use Real-Life Examples: If asked about DI, describe a real-world scenario where you’ve applied it, like injecting a service to handle logging or database connections.
• Show Your Understanding of Lifecycle: Make sure you understand the lifecycle of services (transient, scoped, singleton) and how it affects object creation and memory management.
• Mention Testing: Emphasize how DI makes it easier to mock services and write unit tests for components in isolation.
• Discuss the DI Container: Be prepared to explain how services are added to the DI container and how the container manages the lifetime and scope of services.

Frequently Asked Questions (FAQ)