Overview
Dependency Injection (DI) is a design pattern in .NET development that allows for the creation of loosely coupled code. It is a technique where one object supplies the dependencies of another object. This pattern is crucial in .NET development for enhancing code maintainability, enabling easier unit testing, and allowing for greater flexibility and scalability in applications.
Key Concepts
- IoC Containers: Frameworks that manage the creation and life cycle of objects and their dependencies.
- Service Lifetimes: How long an instance of a service is alive (Transient, Scoped, Singleton).
- Constructor Injection: The most common form of DI, where dependencies are provided through a class's constructor.
Common Interview Questions
Basic Level
- What is Dependency Injection and why is it useful in .NET applications?
- How do you implement constructor injection in a .NET class?
Intermediate Level
- Can you explain the different service lifetimes in .NET Core DI and their use cases?
Advanced Level
- How can you implement a custom IoC container in .NET, and why might you choose to do so?
Detailed Answers
1. What is Dependency Injection and why is it useful in .NET applications?
Answer: Dependency Injection (DI) is a design pattern used to achieve Inversion of Control (IoC) between classes and their dependencies. In .NET applications, DI helps in reducing tight coupling between software components, making the system more modular, easier to test, maintain, and extend. By delegating the responsibility of creating dependencies to an external source, applications become more flexible and decoupled.
Key Points:
- DI promotes loose coupling between objects, enhancing code maintainability.
- It simplifies unit testing by allowing dependencies to be mocked or stubbed.
- DI is facilitated in .NET applications using the built-in IoC container or third-party containers like Autofac, Unity, etc.
Example:
public interface IMessageService
{
void Send(string message);
}
public class EmailService : IMessageService
{
public void Send(string message)
{
// Implementation to send email
Console.WriteLine("Sending email: " + message);
}
}
public class NotificationService
{
private readonly IMessageService _messageService;
// Constructor injection
public NotificationService(IMessageService messageService)
{
_messageService = messageService;
}
public void Notify(string message)
{
_messageService.Send(message);
}
}
// In application setup
var notificationService = new NotificationService(new EmailService());
notificationService.Notify("Hello DI!");
2. How do you implement constructor injection in a .NET class?
Answer: Constructor injection in a .NET class is implemented by defining a constructor that takes the dependencies as parameters. The .NET framework or IoC container then provides these dependencies when the class is instantiated.
Key Points:
- Constructor injection ensures that the class has all necessary dependencies it needs to function.
- It makes the class's dependencies explicit and promotes immutability.
- This approach is the most common and recommended way to achieve DI in .NET.
Example:
public class OrderService
{
private readonly IProductRepository _productRepository;
private readonly IOrderRepository _orderRepository;
// Constructor injection
public OrderService(IProductRepository productRepository, IOrderRepository orderRepository)
{
_productRepository = productRepository;
_orderRepository = orderRepository;
}
public void ProcessOrder(Order order)
{
// Use the injected dependencies to process the order
}
}
3. Can you explain the different service lifetimes in .NET Core DI and their use cases?
Answer: In .NET Core DI, there are three primary service lifetimes: Transient, Scoped, and Singleton.
- Transient: Services are created each time they are requested. This lifetime works best for lightweight, stateless services.
- Scoped: Services are created once per client request (connection). It is useful for services that need to maintain state within a single request.
- Singleton: A single instance of the service is created and shared throughout the application's lifetime. Singleton services are ideal for shared resources or configurations.
Key Points:
- Choosing the correct service lifetime is crucial for proper resource management and performance.
- Scoped services can be used to share data within a request, avoiding global state.
- Singleton services must be designed for concurrency and must be thread-safe.
Example:
public void ConfigureServices(IServiceCollection services)
{
services.AddTransient<ITransientService, TransientService>();
services.AddScoped<IScopedService, ScopedService>();
services.AddSingleton<ISingletonService, SingletonService>();
}
4. How can you implement a custom IoC container in .NET, and why might you choose to do so?
Answer: Implementing a custom IoC container in .NET involves creating a class to manage object creation, lifecycle, and dependencies. While .NET's built-in IoC container or popular third-party containers like Autofac or Unity usually suffice, certain scenarios might require a custom container for more control or specific optimizations.
Key Points:
- Custom IoC containers can offer tailored solutions for unique requirements.
- They can be optimized for specific scenarios, potentially improving performance.
- Developing a custom IoC container requires a deep understanding of DI principles and patterns.
Example:
public class CustomContainer
{
private readonly Dictionary<Type, Func<object>> _registrations = new();
public void Register<TService, TImplementation>()
{
_registrations.Add(typeof(TService), () => Activator.CreateInstance(typeof(TImplementation)));
}
public TService Resolve<TService>()
{
return (TService)_registrations[typeof(TService)]();
}
}
// Usage
var container = new CustomContainer();
container.Register<IMessageService, EmailService>();
var service = container.Resolve<IMessageService>();
By understanding and utilizing the principles of Dependency Injection, developers can create more modular, testable, and maintainable .NET applications.
