For seasoned developers looking to showcase their advanced skills in .NET Core, this article compiles 20 in-depth interview questions and answers. Covering topics like custom middleware, gRPC, async streams, and performance optimization, this guide is tailored to help you demonstrate your expertise and tackle challenging technical interviews with confidence.

1. What is the purpose of the IHostedService interface in .NET Core, and how do you implement a background service using it?

   Answer:

   The IHostedService interface is used to create long-running background services in .NET Core applications.

   Explanation:

   Implementing IHostedService allows you to run background tasks within a hosted environment. You need to implement the StartAsync and StopAsync methods. To register the service, you add it in ConfigureServices using services.AddHostedService<MyService>(). This is useful for tasks like polling, consuming messages, or scheduled jobs without blocking the main thread.

2. Explain the difference between app.Use, app.UseMiddleware, and app.Run methods in the ASP.NET Core middleware pipeline.

   Answer:

   • app.Use: Adds middleware that can process requests and pass them to the next component.

   • app.UseMiddleware<T>: Injects custom middleware into the pipeline via a middleware class.

   • app.Run: Adds terminal middleware that processes requests without calling the next component.

Explanation:

• app.Use is for middleware that needs to perform actions both before and after the next middleware.

• app.UseMiddleware<T> is a generic method to add custom middleware classes.

• app.Run is used when no further middleware should handle the request, effectively ending the pipeline.

3. What are Async Streams in C#, and how do they differ from regular asynchronous methods?

   Answer:

   Async Streams allow asynchronous iteration over a collection using IAsyncEnumerable<T> and await foreach.

   Explanation:

   Async Streams enable methods to produce a stream of data asynchronously. Unlike regular async methods that return a single Task, async streams can yield multiple values over time, improving responsiveness and resource utilization when working with data sequences.

4. How do you implement custom middleware in ASP.NET Core?

   Answer:

   By creating a class with an Invoke or InvokeAsync method that accepts an HttpContext, and registering it with app.UseMiddleware<T>().

   Explanation:

   Custom middleware handles HTTP requests and responses. It should accept a RequestDelegate in its constructor and use HttpContext to access request and response data. Register the middleware in the Configure method to integrate it into the pipeline.

5. What is gRPC, and how is it implemented in .NET Core?

   Answer:

   gRPC is a high-performance, open-source RPC framework using Protocol Buffers, implemented in .NET Core with the Grpc.AspNetCore package.

   Explanation:

   gRPC enables efficient communication between services. In .NET Core, you define services and messages in .proto files. The tooling generates server and client code. You configure gRPC services in Startup.cs and use them for inter-service communication in microservices architectures.

6. Explain the use of Span<T> and Memory<T> in .NET Core.

   Answer:

   Span<T> and Memory<T> are types for representing contiguous regions of memory, allowing high-performance, zero-allocation code.

   Explanation:

   • Span<T>: A stack-only type for accessing memory, including arrays and unmanaged memory, without copying.

   • Memory<T>: Similar to Span<T>, but can be stored on the heap and used asynchronously. They help in reducing memory allocations and improving performance, especially in applications processing large amounts of data.

7. What are the differences between Task, ValueTask, and IAsyncEnumerable in .NET Core?

   Answer:

   • Task: Represents an asynchronous operation returning a single value.

   • ValueTask: A lightweight alternative to Task that can avoid heap allocations.

   • IAsyncEnumerable: Represents an asynchronous stream of values.

Explanation:

ValueTask improves performance when the result is already available or cached. IAsyncEnumerable is used for asynchronous iteration over collections, allowing data to be processed as it's received.

8. What is the difference between ConfigureServices and ConfigureContainer in ASP.NET Core?

   Answer:

   ConfigureServices is used to register services with the built-in DI container, while ConfigureContainer allows you to integrate a third-party DI container.

   Explanation:

   If you need features beyond the built-in container, such as property injection or advanced scoping, you can use ConfigureContainer to set up an external container like Autofac, providing more control over dependency resolution.

9. How do you implement Health Checks in ASP.NET Core applications?

   Answer:

   By adding health check services with services.AddHealthChecks() and mapping health check endpoints using app.UseEndpoints.

   Explanation:

   Health checks monitor the health of application components (e.g., database, external services). They can be exposed via endpoints and integrated with monitoring systems to ensure application reliability.

10. Explain the concept of dependency injection scopes and how they can affect ASP.NET Core applications.

    Answer:

    Dependency injection scopes determine the lifecycle of service instances: Singleton, Scoped, and Transient.

    Explanation:

    • Singleton: One instance for the application's lifetime.

    • Scoped: One instance per request.

    • Transient: A new instance each time it's requested. Misconfigured scopes can lead to issues like unintended shared state or resource leaks, especially when injecting scoped services into singletons.

11. What is the IHttpClientFactory, and why should you use it in .NET Core?

    Answer:

    IHttpClientFactory manages the creation and configuration of HttpClient instances, promoting reuse and avoiding common pitfalls.

    Explanation:

    It prevents socket exhaustion by managing handler lifetimes, allows named clients with specific configurations, and centralizes HTTP client settings, improving reliability and testability.

12. How do you implement global exception handling in ASP.NET Core?

    Answer:

    By using the UseExceptionHandler middleware or writing custom middleware to catch and handle exceptions application-wide.

    Explanation:

    Global exception handling ensures that unhandled exceptions are caught, logged, and appropriate responses are returned to clients, enhancing security and user experience.

13. What is SignalR in ASP.NET Core, and how does it facilitate real-time communication?

    Answer:

    SignalR is a library for adding real-time web functionality, allowing server-side code to push content to connected clients instantly.

    Explanation:

    It abstracts underlying transport mechanisms like WebSockets, enabling features like live chat, notifications, and real-time updates with minimal code changes.

14. Explain how C# 8.0 nullable reference types help prevent null reference exceptions.

    Answer:

    Nullable reference types enable the compiler to recognize variables that may be null, enforcing null checks at compile time.

    Explanation:

    By annotating reference types with ?, the compiler warns when nullable variables are dereferenced without null checks, reducing runtime exceptions and improving code safety.

15. What is the role of the GCSettings.LatencyMode property, and when would you adjust it?

    Answer:

    GCSettings.LatencyMode adjusts the garbage collector's behavior to optimize for latency or throughput.

    Explanation:

    In scenarios requiring minimal interruptions (e.g., real-time applications), setting LatencyMode to LowLatency reduces GC pauses. However, it may increase memory usage, so it's a trade-off.

16. How does the .NET Core runtime handle assembly loading and resolving dependencies?

    Answer:

    The runtime uses AssemblyLoadContext to manage assembly loading, allowing for isolation and dynamic loading of assemblies.

    Explanation:

    Custom AssemblyLoadContext instances enable loading assemblies into separate contexts, useful for plugin systems or applications requiring multiple versions of an assembly without conflicts.

17. What are ref structs, and why would you use them in .NET Core?

    Answer:

    ref structs are stack-only types that cannot be boxed, ensuring high performance and preventing heap allocations.

    Explanation:

    Used in performance-critical code, ref structs like Span<T> avoid the overhead of heap allocation and garbage collection, improving efficiency in scenarios like parsing or buffer manipulation.

18. Explain how to create a custom JsonConverter with System.Text.Json in .NET Core.

    Answer:

    By deriving from JsonConverter<T> and overriding the Read and Write methods to control serialization behavior.

    Explanation:

    Custom converters handle complex serialization scenarios, such as polymorphic types or custom formats. They are registered with JsonSerializerOptions and enable fine-grained control over JSON processing.

19. How do you handle versioning in ASP.NET Core Web APIs?

    Answer:

    By using API versioning techniques like URL segmenting, query parameters, or headers, often facilitated by the Microsoft.AspNetCore.Mvc.Versioning package.

    Explanation:

    API versioning allows you to introduce changes without breaking existing clients. It supports multiple API versions side by side, ensuring backward compatibility.

20. What is the IOptionsMonitor<T> interface, and how does it differ from IOptions<T>?

    Answer:

    IOptionsMonitor<T> provides access to configuration options and supports change notifications, whereas IOptions<T> provides a snapshot of options.

    Explanation:

    IOptionsMonitor<T> is useful when options can change at runtime, as it automatically updates bound objects and allows you to react to changes via callbacks, enhancing application flexibility.

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These advanced .NET Core interview questions and answers offer concise explanations of complex topics, facilitating efficient preparation and a deeper understanding of the framework's capabilities