Top 50 .NET Core Interview Questions & Answers


✅ Top 50 .NET Core Interview Questions & Answers

(with Definition + Details + Real Example + Syntax + Code)

1. What is .NET Core?

Definition: .NET Core is a free, open-source, cross-platform framework maintained by Microsoft. It allows developers to build web apps, APIs, microservices, cloud-native solutions, desktop, and mobile applications.

Details: It is modular, lightweight, high-performance, and can run on Windows, Linux, and macOS.

Real-Time Example: A bank runs its customer portal on Linux servers using .NET Core.

Syntax:

dotnet new webapi -n BankAPI

2. What are the advantages of .NET Core?

Definition: Benefits of using .NET Core compared to traditional .NET Framework.

Details:

  • Cross-platform

  • High performance with Kestrel server

  • Supports microservices and containers (Docker, Kubernetes)

  • Built-in Dependency Injection

  • Unified framework for Web, API, and MVC

Real-Time Example: An e-commerce site migrates from .NET Framework to .NET Core to deploy on Linux-based AWS servers.

3. What is Kestrel?

Definition: Kestrel is the default cross-platform web server in ASP.NET Core.

Details: It is lightweight and very fast, designed to handle high loads. Often runs behind a reverse proxy like IIS or Nginx.

Real-Time Example: An e-commerce site handles thousands of concurrent users using Kestrel + Nginx.

Code Example:

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseKestrel();
            webBuilder.UseStartup<Startup>();
        });

4. What is Middleware?

Definition: Middleware are software components that handle requests/responses in the ASP.NET Core pipeline.

Details: Middleware executes in sequence and can modify requests before reaching the controller or modify responses before returning.

Real-Time Example: Authentication middleware validates JWT tokens before reaching APIs.

Code Example:

app.Use(async (context, next) =>
{
    Console.WriteLine("Request started");
    await next();
    Console.WriteLine("Response completed");
});

5. Difference between .NET Core and .NET 5/6/7?

Definition:

  • .NET Core existed till version 3.1

  • From .NET 5, Microsoft merged everything into .NET Unified Platform

Details:

  • .NET 5+ includes support for desktop, mobile, cloud, and gaming in one platform.

  • Regular updates with Long Term Support (LTS).

Real-Time Example: Companies migrate apps from .NET Framework/Core to .NET 6 (LTS) for better support.

6. What is Dependency Injection (DI)?

Definition: DI is a design pattern where dependencies are provided instead of being created inside a class.

Details: ASP.NET Core has built-in DI with AddSingleton, AddScoped, and AddTransient.

Real-Time Example: Injecting a logging service into multiple controllers.

Code Example:

public interface IMessage { string GetMessage(); }
public class HelloMessage : IMessage { public string GetMessage() => "Hello"; }

services.AddScoped<IMessage, HelloMessage>();

public class HomeController : Controller
{
    private readonly IMessage _msg;
    public HomeController(IMessage msg) { _msg = msg; }
    public IActionResult Index() => Content(_msg.GetMessage());
}

7. What are Controllers?

Definition: Controllers handle HTTP requests and return responses (JSON, HTML, etc.).

Details: Controllers usually inherit from ControllerBase or Controller.

Real-Time Example: A ProductController serves /api/products.

Code Example:

[Route("api/[controller]")]
public class ProductController : ControllerBase
{
    [HttpGet]
    public IActionResult Get() => Ok(new[] { "Laptop", "Phone" });
}

8. What is Startup.cs?

Definition: Startup.cs configures services and middleware.

Details: Contains two methods:

  • ConfigureServices() → register services

  • Configure() → setup request pipeline

Real-Time Example: Register EF Core, Authentication, and CORS policies here.

Code Example:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();
}
public void Configure(IApplicationBuilder app)
{
    app.UseRouting();
    app.UseEndpoints(endpoints => endpoints.MapControllers());
}

9. What is EF Core?

Definition: EF Core is an ORM (Object Relational Mapper) for database operations.

Details: Supports LINQ queries, migrations, multiple databases (SQL Server, MySQL, PostgreSQL).

Real-Time Example: A payroll app uses EF Core to fetch employee salary from SQL Server.

Code Example:

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
}
public class AppDbContext : DbContext
{
    public DbSet<Employee> Employees { get; set; }
}

10. What are Migrations in EF Core?

Definition: Migrations update database schema from model classes.

Details: dotnet ef command line is used for migrations.

Real-Time Example: Adding a new Salary column to Employee table.

Syntax:

dotnet ef migrations add AddSalaryColumn
dotnet ef database update

11. What is Singleton, Scoped, Transient in DI?

Definition:

  • Singleton → one instance for whole app

  • Scoped → one instance per request

  • Transient → new instance every time

Real-Time Example:

  • Singleton: Logger

  • Scoped: User session

  • Transient: Random number generator

Code Example:

services.AddSingleton<ILog, LogService>();
services.AddScoped<IUserService, UserService>();
services.AddTransient<IEmailService, EmailService>();

12. How does Logging work in .NET Core?

Definition: Logging helps track errors, info, and warnings.

Details: Built-in providers: Console, Debug, EventLog, Azure.

Real-Time Example: Log every failed payment attempt.

Code Example:

public class HomeController : Controller
{
    private readonly ILogger<HomeController> _logger;
    public HomeController(ILogger<HomeController> logger) => _logger = logger;
    public IActionResult Index()
    {
        _logger.LogInformation("Page Loaded");
        return View();
    }
}

13. What are Filters?

Definition: Filters are used for cross-cutting concerns (logging, caching, auth).

Details: Types → Action, Exception, Authorization, Resource filters.

Real-Time Example: Apply a filter for logging all API requests.

Code Example:

public class LogActionFilter : IActionFilter
{
    public void OnActionExecuting(ActionExecutingContext context) =>
        Console.WriteLine("Before action");
    public void OnActionExecuted(ActionExecutedContext context) =>
        Console.WriteLine("After action");
}

14. What is Routing?

Definition: Routing decides which controller/action handles a request.

Details: Two types:

  • Convention-based

  • Attribute-based

Real-Time Example: /products/10 → ProductController.Get(10)

Code Example:

app.UseEndpoints(endpoints =>
{
    endpoints.MapControllerRoute(
        name: "default",
        pattern: "{controller=Home}/{action=Index}/{id?}");
});

15. How do you handle Global Exceptions?

Definition: Global exception handling catches errors at application level.

Details: Use app.UseExceptionHandler() or custom middleware.

Real-Time Example: Show a friendly error page if DB connection fails.

Code Example:

app.UseExceptionHandler("/Home/Error");

16. What is gRPC?

Definition: gRPC is a high-performance Remote Procedure Call framework.

Details: Uses HTTP/2 and Protobuf (binary format).

Real-Time Example: Used in microservices communication in banking systems.

Code Example:

services.AddGrpc();
app.UseEndpoints(endpoints =>
{
    endpoints.MapGrpcService<MyGrpcService>();
});

17. Difference between Web API in .NET Framework vs Core?

Definition:

  • .NET Framework Web API → Windows only, heavy, separate from MVC.

  • .NET Core API → Lightweight, cross-platform, unified with MVC.

Real-Time Example: APIs migrated to Core for Docker deployment.

Code Example:

dotnet new webapi

18. How to implement Caching?

Definition: Caching stores frequently used data in memory.

Details: Supports In-Memory, Distributed, and Response caching.

Real-Time Example: Cache product catalog to avoid frequent DB calls.

Code Example:

services.AddMemoryCache();
if (!_cache.TryGetValue("time", out string currentTime))
{
    currentTime = DateTime.Now.ToString();
    _cache.Set("time", currentTime, TimeSpan.FromMinutes(1));
}

19. What is JWT Authentication?

Definition: JWT (JSON Web Token) is a secure, token-based authentication method.

Details: Token contains user claims, signed using a secret key.

Real-Time Example: A mobile app uses JWT token for login and API calls.

Code Example:

services.AddAuthentication(JwtBearerDefaults.AuthenticationScheme)
.AddJwtBearer(options =>
{
    options.TokenValidationParameters = new TokenValidationParameters
    {
        ValidateIssuer = true,
        ValidateAudience = true,
        IssuerSigningKey = new SymmetricSecurityKey(
            Encoding.UTF8.GetBytes("MySecretKey"))
    };
});

20. What are Health Checks?

Definition: Health checks verify if the app, database, or services are working.

Details: Expose /health endpoint for monitoring tools.

Real-Time Example: Kubernetes checks health endpoint before routing traffic.

Code Example:

services.AddHealthChecks();
app.UseEndpoints(endpoints =>
{
    endpoints.MapHealthChecks("/health");
});

21. What are Tag Helpers in ASP.NET Core?

Definition: Tag helpers allow server-side code to participate in creating HTML.

Details: Strongly typed, IntelliSense-supported.

Real-Time Example: Use asp-for to bind a model property to input.

Code Example:

<input asp-for="Name" class="form-control" />

22. What is ViewComponent in ASP.NET Core?

Definition: ViewComponents are like partial views but with logic.

Details: Used for reusable UI sections.

Real-Time Example: Sidebar showing recent blog posts.

Code Example:

public class RecentPostsViewComponent : ViewComponent
{
    public IViewComponentResult Invoke() => View("Default", new[] { "Post1", "Post2" });
}

23. What is SignalR?

Definition: SignalR is a library for real-time communication (WebSockets).

Details: Enables server → client push.

Real-Time Example: Chat apps, stock price updates.

Code Example:

services.AddSignalR();
app.UseEndpoints(endpoints =>
{
    endpoints.MapHub<ChatHub>("/chatHub");
});

24. What is Minimal API in .NET 6+?

Definition: Minimal APIs allow creating lightweight APIs without full controllers.

Details: Simple syntax using lambda expressions.

Real-Time Example: Microservices exposing quick APIs.

Code Example:

var app = WebApplication.CreateBuilder(args).Build();
app.MapGet("/hello", () => "Hello World");
app.Run();

25. What is Cross-Origin Resource Sharing (CORS)?

Definition: CORS allows APIs to be accessed from different domains.

Details: Important for SPAs (React, Angular).

Real-Time Example: React app calls ASP.NET Core API.

Code Example:

services.AddCors(options =>
{
    options.AddPolicy("AllowAll", builder =>
        builder.AllowAnyOrigin().AllowAnyMethod().AllowAnyHeader());
});
app.UseCors("AllowAll");


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