Here are common interview questions related to Dependency Injection (DI) in .NET Core. Preparing for a job interview can be daunting, especially when it comes to technical topics like Dependency Injection. To help you ace your next interview, we’ve compiled a list of 25 essential Dependency Injection interview questions and answers. These questions will not only test your knowledge but also give you a deeper understanding of this crucial software design pattern.
These dependency injection Interview questions in .NET Core help you to chase your next .NET interview. Dependency Injection is a crucial topic in ASP.NET Core from an interview perspective.
Also, check out the other Interview Questions & Answers Set:-
- Interview Set 1: 10 .NET Core Interview Questions & Answers.
- Interview Set 2: .NET 8 & .NET 9 Interview Questions and Answers.
- Interview Set 3: Top 10 .NET Interview Questions on CLR, CTS, and CLS (With Detailed Answers)
- Interview Set 4: Middleware Interview Questions in ASP.NET Core
🔹 What is Dependency Injection?
Dependency Injection (DI) is a design pattern that allows an object to receive its dependencies from an external source rather than creating them itself.
- Without DI: Classes are tightly coupled and harder to test.
- With DI: Classes are loosely coupled, easier to maintain, and testable.
In .NET Core, DI is built-in, making it the backbone of modern, scalable applications.
1. What is Dependency Injection and why is it important?
Dependency Injection is the process of supplying dependencies (objects a class needs) from outside.
Importance:
- Promotes loose coupling
- Improves testability with mocks/stubs
- Makes code maintainable and reusable
- Encourages separation of concerns
2. Explain the difference between IoC and DI.
- IoC (Inversion of Control): A principle where control of creating and managing dependencies is inverted from the class to a container/framework.
- DI (Dependency Injection): A technique to achieve IoC by injecting dependencies (via constructor, method, or property).
So, DI is a way to implement IoC.
3. What are the different lifetimes in .NET DI?
.NET Core provides three built-in lifetimes:
- Singleton → Created once and shared across the app.
- Scoped → Created once per request.
- Transient → Created every time it is requested.
4. How does .NET Core implement DI out of the box?
.NET Core includes a lightweight IoC container in Microsoft.Extensions.DependencyInjection.
Example in Program.cs:
builder.Services.AddSingleton<IMyService, MyService>();
builder.Services.AddScoped<IOrderService, OrderService>();
builder.Services.AddTransient<ILoggerService, LoggerService>();
5. Can we replace the built-in IoC container? Why would we?
Yes ✅ You can replace the default container with others like Autofac, Unity, Castle Windsor.
Reasons:
- Advanced features (e.g., property injection, decorators)
- Better performance for large projects
- More flexibility in object graph resolution
6. Give a real-world example where DI improves testability.
Imagine a class OrderProcessor that depends on IPaymentService.
Without DI → Directly creates PaymentService (hard to test).
With DI → Injects IPaymentService, allowing mocks in unit tests.
public class OrderProcessor
{
private readonly IPaymentService _paymentService;
public OrderProcessor(IPaymentService paymentService)
{
_paymentService = paymentService;
}
}7. What is the difference between Constructor Injection, Property Injection, and Method Injection?
- Constructor Injection → Dependencies passed via constructor (most common in .NET Core).
- Property Injection → Dependencies assigned via public properties.
- Method Injection → Dependencies passed as method parameters.
8. What happens if multiple services are registered for the same interface?
By default, the last registered service wins when resolving with GetService<T>().
You can also resolve multiple implementations using:
IEnumerable<IMyService> services
9. How can you inject configuration and options using DI?
.NET Core provides IOptions<T> pattern:
builder.Services.Configure<MySettings>(builder.Configuration.GetSection("MySettings"));
Injected into class:
public class MyService
{
private readonly MySettings _settings;
public MyService(IOptions<MySettings> options)
{
_settings = options.Value;
}
}
10. What are service descriptors in .NET DI?
Each registered service has a ServiceDescriptor with:
- ServiceType (interface/class)
- ImplementationType (actual class)
- Lifetime (Singleton/Scoped/Transient)
11. Can you inject services into static classes?
No ❌ Static classes cannot use DI directly because they don’t support constructors.
Solution → Use service locator pattern or wrap logic inside injectable classes.
12. How does DI improve microservices architecture?
- Encourages modularity
- Makes services independent and testable
- Easier to replace implementations without breaking other parts
- Works well with cloud patterns like Scoped services per request
13. What is the difference between AddSingleton, AddScoped, and AddTransient?
- AddSingleton → One instance for the app lifetime.
- AddScoped → One instance per HTTP request.
- AddTransient → New instance every time requested.
Example:
builder.Services.AddSingleton<ILoggerService, LoggerService>();
builder.Services.AddScoped<IUserService, UserService>();
builder.Services.AddTransient<INotificationService, NotificationService>();
14. Can you inject multiple implementations of the same interface?
Yes ✅ Register multiple services:
builder.Services.AddTransient<INotifier, EmailNotifier>();
builder.Services.AddTransient<INotifier, SmsNotifier>();
Then inject:
public MyService(IEnumerable<INotifier> notifiers) { ... }
15. What are best practices for Dependency Injection in .NET Core?
- Use constructor injection (preferred).
- Keep constructors short (avoid too many dependencies → refactor).
- Register services with correct lifetime.
- Avoid service locator pattern unless necessary.
- Use interfaces for testability.
- Replace built-in container only if required.
📝 Conclusion
Dependency Injection in .NET Core is one of the most important concepts for building scalable, testable, and maintainable applications.
👉 Key takeaways:
- DI promotes loose coupling and testability.
- .NET Core supports Singleton, Scoped, and Transient lifetimes.
- The built-in DI container is lightweight and extensible, but you can use Autofac, Unity, or Castle Windsor for advanced scenarios.
- DI supports constructor, property, and method injection, but constructor injection is most common.
- Real-world applications, especially microservices, heavily rely on DI for better architecture.
With these 15 Dependency Injection interview questions and answers, you’ll be ready to tackle both theory and coding questions in your next .NET interview. 🚀
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