Basic :

1. What types of Web APIs are most commonly used?

Answer :   The most commonly used Web API types are:

•  REST (Representational State Transfer) – Lightweight, uses HTTP methods, JSON/XML, and is the most popular.

• SOAP (Simple Object Access Protocol) – XML-based, follows strict standards, and provides high security.

• GraphQL – Client-driven queries that avoid over-fetching and under-fetching data.

• gRPC – High-performance RPC framework using HTTP/2 and Protobuf.

  REST APIs are the most widely used due to their simplicity and scalability.

2. How do RESTful APIs manage state?

Answer : 

• Each request contains all required information.

• The server does not store client session data.

• Authentication is handled using tokens such as JWT or OAuth with every request.

3. What is the difference between PUT and POST methods in a Web API?

Answer : 

POST

The POST method is used to submit an entity to a resource. It is used for creating a new resource or adding a new entity to an existing resource.

PUT

The PUT method is used to update a resource or create a new resource if it does not exist. When a PUT request is sent, the entire resource is replaced with the new data. If the resource does not exist, it will be created.

4. How do Web APIs implement caching mechanisms?

Answer : 

Caching improves performance by reducing server load.

Common techniques include:

• HTTP headers 

• In-memory caching

• Distributed caching (Redis)

• Response caching middleware.

Caching avoids repeated database calls for the same response data.

5. What are the common tools for testing Web APIs?

Answer : 

• Postman

• Swagger (OpenAPI)

• Fiddler

6. What is an API gateway?

Answer : 

An API Gateway is a single entry point for multiple APIs.

• Authentication and authorization

• Rate limiting and throttling

• Logging and monitoring

• Load balancing

7. How do Web APIs use OAuth for security?

Answer :

• User authenticates with the Authorization Server

• Client receives an access token

• Token is sent in the API request header

• API validates the token

8. How do you optimize the performance of a Web API?

Answer : 

• Using async/await.

• Enabling response caching.

• Using pagination.

• Minimizing payload size.

• Optimizing database queries.

• Using CDN for static data.

9. How does a Web API use tokens for authentication?

Answer : 

• User logs in

• Server generates a token (JWT)

• Token is stored on the client

• Token is sent in the Authorization  header

• Server validates the token

10. What is API throttling ?

Answer : API throttling limits the number of requests from a client.

Purpose:

• Prevent abuse

• Improve performance

• Ensure fair usage

11. How do routing and URL mapping work in ASP.NET Web API?

Answer : Routing maps incoming URLs to controller actions.

Types:

• Convention-based routing.

• Attribute routing.

12. How is data serialization handled in ASP.NET Web API?

Answer :  ASP.NET Web API automatically serializes objects to:

• JSON

• XML 

Uses:

• System.Text.Json

• Newtonsoft.Json

13. How do you secure a Web API in ASP.NET?

Answer :

• HTTPS

• JWT authentication

• OAuth 

• Role-based authorization

• Input validation

• CORS configuration

14.How do you handle CORS (Cross-Origin Resource Sharing) in ASP.NET Web API?

Answer :  CORS is used when a frontend application hosted on one domain needs to access a Web API hosted on another domain. By default, browsers block such cross-origin requests for security reasons.

15. How is asynchronous programming managed in ASP.NET Web API?

Answer : 

Asynchronous programming uses async and await.

Benefits:

• Non-blocking requests

• Better scalability

• Improved performance

16. How do you handle large file uploads and downloads using ASP.NET Web API?

Answer :

• Streaming uploads.

• Multipart/form-data.

• Increasing request size limits.

17. Can we create SOAP based message using Web API?

Answer :  Yes, but ASP.NET Web API is mainly REST-focused.

• Custom XML serialization

• Manual SOAP envelope handling

18. What are the Advantages of Using ASP.NET Web API?

Answer : 

• Lightweight and fast.

• RESTful architecture

• Platform independent.

• Built-in dependency injection.

• Supports JSON and XML.

19. Serialization and Deserialization in Web API?

Answer : 

• Serialization: Converts objects into JSON or XML

• Deserialization: Converts JSON or XML back into objects

20. How does ASP.NET Web API handle model binding and validation?

Answer :

• Model binding maps request data to action parameters

• Validation uses data annotations

21. What is Dependency Injection in Web API and why is it important?

Answer :

Dependency Injection (DI) provides required services automatically.

Benefits:

• Loose coupling

• Better maintainability

ASP.NET Core has a built-in DI container.

22. What are DTOs (Data Transfer Objects) and why are they used in Web API?

answer :

• Transfer data between layers

• Prevent over-exposing domain models

• Improve security and performance

23. How do you use attribute routing in C# Web API?

Answer : Attribute routing defines routes at the method level.

24. How can you implement caching in Web API?

Answer :

• Response caching

• In-memory caching

• Distributed caching

25. How can you implement logging in an ASP.NET Web API application?

Answer : Logging helps track errors and performance.

• Serilog

• NLog

Advanced :

26. What are the main return types supported in Web API?

Answer :

1. Primitive types – int , string and bool.

2. Complex objects – Custom classes or models

3. IHttpActionResult – Preferred, flexible, clean

4. HttpResponseMessage – Full control over response.

27 . What are media type formatters?

Answer : Media type formatters convert .NET objects into response formats and vice versa. 

• JSON Formatter – Converts data to JSON

• XML Formatter – Converts data to XML

28. What is SOAP?

Answer : SOAP (Simple Object Access Protocol) is a protocol for exchanging structured data using XML.

• Uses XML only

• Mostly used in enterprise applications

29. Describe the meaning of exception filters.

Answer : Exception filters handle errors that occur in Web API.

• Log errors

• Return user-friendly error responses

30. Can the Web API return View?

Answer : No.  Web API return only data. MVC return view

31. What are HTTP status codes, exactly?

Answer :

• 200 OK – Success

• 201 Created – Resource created

• 400 Bad Request – Client error

• 401 Unauthorized – Authentication required

• 404 Not Found – Resource not found

• 500 Internal Server Error – Server error

32. How is ASP.Net’s Basic Authentication implemented in the Web API?

Answer :

• Client sends username and password in request headers

• Server validates credentials on every request.

33. What are the different HTTP method types that may be used with Web API?

Answer :

• GET – Retrieve data

• POST – Create data

• PUT – Update full data

• PATCH – Update partial data

• DELETE – Remove data

34. What do you mean by Parameter Binding in ASP.NET?

Answer : Parameter binding maps incoming request data to action method parameters.

• URL

• Query string

• Headers

35. What does Web API CORS mean?

Answer : CORS (Cross-Origin Resource Sharing) allows requests from different domains.

36. What do you mean by Web API Routing?

Answer : Routing defines how URLs map to controller actions.

Example :     /api/users/10

37. How does one handle errors in Web API?

Answer :

• Try-catch blocks

• Exception filters

• Global exception handling

• Logging tools (Serilog, NLog)

38. What are the key components of a Web API?

Answer :

• Controllers

• Routing

• Models

• Authentication & Authorization

39. What is RESTful API?

Answer : A RESTful API follows REST principles:

• Stateless communication

• Uses standard URLs

• Returns JSON/XML

40. Explain the difference between GET and POST requests.

Answer :

GET :

1. Fetch Data
2. Less secure
3. Data in URL

POST :

1. Send data
2. Data in body
3. More secure

41. What are differences between JSON and XML?

Answer :

JSON :

• Lightweight
• Faster
• Easy to read

XML :

• Heavy
• Slower
• Less use

42. Explain the concept of rate limiting in Web APIs.

Answer : Rate limiting controls how many requests a client can make.

It prevents:

• Server overload

• DDoS attacks

43. What is content negotiation in Web APIs?

Answer : Content negotiation decides response format based on client request.

• Client asks for JSON → API returns JSON

• Client asks for XML → API returns XML

44. What are the advantages of using JWT for authentication in Web APIs?

Answer :

• Stateless

• Fast

• Secure when used with HTTPS

45. What are the common authentication methods used in Web APIs, and when would you use each?

Answer :

• Basic Auth   –>    Simple Internal App
• JWT              –>    Modern APIs, mobile apps
• OAuth           –>    Third-party access
• API Keys       –>    Public APIs

46. What is MVC? Write difference between MVC and Web API?

Answer :

MVC :

• Return View
• focused on UI
• browser based

Web API :

• Return Data 
• focused on API
• Any Client 

47. Who can consume Web API?

Answer :

• Web applications

• Mobile apps

• Desktop apps

• Third-party services

48. What are Web API filters?

Answer :

• Authorization

• Logging

• Error handling

• Validation

49. What is the difference between Web API and WCF?

Answer : 

Web API :

• Light weight
• JSON support
• easy to use

WCF :

• Heavy
• SOAP Based
• XML focused

50. Why web api ?

Answer : Web API is used because it allows different applications to talk to each other easily.

• Data can be shared securely

• Frontend and backend stay independent

• Performance improves

• Integration becomes simple