Here are the top 25 .NET interview questions and their answers, covering C#, ASP.NET Core, Entity Framework, and related technologies.

1. What is the difference between .NET Framework, .NET Core, and .NET 5/6?

• .NET Framework: Windows-only, mature, not cross-platform.
• .NET Core: Cross-platform, open-source, modular, high performance.
• .NET 5/6+: Unified platform for Windows, Linux, macOS; combines best of both, with new features and performance improvements.

2. Explain the difference between value types and reference types in C#.

• Value types (e.g., int, struct) store data directly, usually on the stack.
• Reference types (e.g., class, string, array) store a reference to the data, which is on the heap.
• Assigning a value type copies the value; assigning a reference type copies the reference.

3. What are async/await and how do they work?

• async and await are used for asynchronous programming.

• async marks a method as asynchronous.

• await pauses execution until the awaited task completes, without blocking the thread.

• Example:

  public async Task<int> GetDataAsync() {
    var result = await SomeApiCall();
    return result;
  }

4. Explain dependency injection in .NET Core.

• Dependency Injection (DI) is a design pattern where dependencies are provided to a class, not created by it.
• .NET Core has built-in DI. Register services in Startup.cs (or Program.cs in .NET 6+), then inject via constructor.
• Improves testability and maintainability.

5. What are extension methods in C#?

• Extension methods add new methods to existing types without modifying their source code.

• Defined as static methods in a static class, with this before the first parameter.

  public static class StringExtensions {
    public static bool IsNullOrEmpty(this string s) => string.IsNullOrEmpty(s);
  }

6. What is the difference between IEnumerable, ICollection, and IList?

Interface	Description
IEnumerable	Basic iteration over a collection
ICollection	Adds count, add, remove, clear
IList	Adds index-based access (get/set by index)

7. Explain Entity Framework Core and how it works.

• EF Core is an ORM (Object-Relational Mapper) for .NET.

• Maps C# classes to database tables.

• Supports LINQ queries, change tracking, migrations, and more.

• Example usage:

  var users = dbContext.Users.Where(u => u.IsActive).ToList();

8. What are delegates and events?

• Delegate: Type-safe function pointer; can reference methods with a specific signature.

• Event: Special delegate for publisher/subscriber scenarios (e.g., UI events).

• Example:

  public delegate void Notify();
  public event Notify OnNotify;

9. Explain garbage collection in .NET.

• Automatic memory management system.
• Frees memory occupied by objects no longer in use.
• Uses generations (0, 1, 2) for optimization.
• Developers can force collection with GC.Collect(), but it’s not recommended.

10. What is middleware in ASP.NET Core?

• Software components in the request pipeline.

• Each middleware can process requests and responses.

• Registered in Startup.cs (or Program.cs in .NET 6+).

• Example:

  app.UseRouting();
  app.UseAuthentication();
  app.UseAuthorization();

11. What is the difference between Task and Thread?

• Thread: OS-level unit of execution.
• Task: Higher-level abstraction for asynchronous operations; can use threads from the thread pool.
• Tasks are preferred for async programming in .NET.

12. Explain LINQ with an example.

• LINQ (Language Integrated Query) allows querying collections using C# syntax.

• Example:

  var evenNumbers = numbers.Where(n => n % 2 == 0).ToList();

13. What are generics in C#?

• Allow classes, interfaces, and methods to operate on types specified by the caller.

• Provide type safety and reusability.

• Example:

  List<int> numbers = new List<int>();

14. What is the difference between abstract class and interface?

Feature	Abstract Class	Interface
Methods	Can have implementations	Only declarations (C# 8+ allows default)
Fields	Can have fields	Cannot have fields
Multiple Inherit.	No	Yes

15. Explain RESTful API best practices in ASP.NET Core.

• Use proper HTTP verbs (GET, POST, PUT, DELETE).
• Return appropriate status codes.
• Use attribute routing.
• Validate input and handle errors gracefully.
• Secure endpoints (e.g., JWT, OAuth).
• Document APIs (Swagger/OpenAPI).

16. What are tuples in C#?

• Tuples are lightweight data structures to group multiple values.

• Example:

  var person = (Name: "Alice", Age: 30);
  Console.WriteLine(person.Name); // Alice

17. Explain JWT authentication.

• JWT (JSON Web Token) is a compact, URL-safe token for authentication.
• Used for stateless authentication in APIs.
• Contains claims, is signed, and can be validated by the server.

18. What are nullable reference types?

• Feature in C# 8+ to distinguish between nullable and non-nullable reference types.

• Helps avoid NullReferenceException.

• Example:

  string? name = null; // nullable
  string name2 = "not null"; // non-nullable

19. What are records in C# 9?

• Immutable reference types for storing data.

• Provide value-based equality.

• Example:

  public record Person(string Name, int Age);

20. Explain global exception handling in ASP.NET Core.

• Use UseExceptionHandler middleware to catch unhandled exceptions.

• Can log errors and return custom error responses.

• Example:

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

21. What is the difference between ConfigureServices and Configure in Startup.cs?

• ConfigureServices: Register services and dependencies (DI container).
• Configure: Set up the HTTP request pipeline (middleware).

22. Explain options pattern in ASP.NET Core.

• Used for accessing and managing configuration settings.

• Bind configuration sections to strongly typed classes.

• Example:

  services.Configure<MyOptions>(Configuration.GetSection("MyOptions"));

23. What is model binding and validation?

• Model binding: Maps HTTP request data to action method parameters.

• Validation: Uses data annotations to validate input.

• Example:

  public IActionResult Post([FromBody] UserModel model) { ... }

24. What is SignalR and where do you use it?

• SignalR is a library for real-time web functionality (e.g., chat, notifications).
• Supports WebSockets, Server-Sent Events, and Long Polling.
• Used in dashboards, live updates, collaborative apps.

25. Explain dependency lifetimes: Transient, Scoped, Singleton.

• Transient: New instance every time requested.
• Scoped: One instance per request.
• Singleton: One instance for the entire application lifetime.

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Tip: Practice these questions and try to implement small code samples for each to deepen your understanding!