Top 25+ Web API Interview Questions and Answers

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A Web API (Application Programming Interface) lets different software systems talk to each other over the internet. It works using HTTP and helps apps share data quickly and easily. The concept of Web APIs dates back to the early 2000s, but it became widely popular with the rise of RESTful APIs, introduced by Roy Fielding in his 2000 doctoral dissertation. Today, Web APIs power everything from mobile apps to cloud platforms. If you are preparing for a backend or full-stack developer role, you will likely face Web API interview questions. This guide covers 25+ commonly asked questions and answers to help you prepare.

Fun Fact – Over 90% of developers use APIs in some form during software development.

Basic Level Web API Interview Questions for Freshers

Here are Web API interview questions and answers designed to help freshers understand core concepts like HTTP methods, routing, and controllers.

1. What is a Web API and how does it work?

A Web API is a framework that allows software systems to communicate over the internet using HTTP. It exposes endpoints through which data can be exchanged, often in JSON or XML format. Clients send requests to these endpoints, and the API responds with the required data or action.

2. Explain the difference between GET and POST methods in HTTP.

GET is used to fetch data from a server without modifying it. It’s simple and cacheable. POST is used to send data to a server, typically to create or update a resource. It isn’t cached and can carry a larger payload.

3. What is routing in Web API and how is it configured?

Routing maps incoming HTTP requests to controller actions. In ASP.NET Core Web API, it’s configured using attribute routing like [HttpGet(“api/products”)] or in Startup.cs using MapControllers() with conventional routing patterns.

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

Common return types include IActionResult, ActionResult<T>, void, and specific types like string or List<T>. IActionResult is often preferred for flexibility and control over status codes and content.

5. How does Web API handle model binding?

Model binding automatically maps HTTP request data to action method parameters. It binds from route values, query strings, headers, or the request body—especially for complex types in POST or PUT methods.

6. What is the purpose of HTTP status codes in a Web API?

HTTP status codes help the client understand the result of the request. For example, 200 means OK, 201 means Created, 400 means Bad Request, and 404 means Not Found.

Intermediate Level Web API Interview Questions

These interview questions in Web API are commonly asked to intermediate-level professionals. 

7. How do you implement token-based authentication in Web API?

Token-based authentication is commonly done using JWT (JSON Web Token). When a user logs in, the server returns a signed token. The client sends this token in the Authorization header for future requests. The server then validates the token before allowing access to secure endpoints.

8. What are action filters and how are they used?

Action filters are used to run custom logic before or after controller actions. You can use built-in filters like [Authorize], or create custom ones by inheriting from ActionFilterAttribute. They’re useful for logging, validation, or modifying responses.

9. How do you version a Web API?

API versioning can be done via URL (e.g., /api/v1/products), query string (?api-version=1.0), or headers. ASP.NET Core supports versioning through the Microsoft.AspNetCore.Mvc.Versioning package. It helps maintain backward compatibility as your API evolves.

10. What is content negotiation in Web API?

Content negotiation selects the format of the response based on the client’s Accept header. For example, a client may request JSON or XML. ASP.NET Core handles this using formatters and returns the appropriate response type automatically.

11. How does attribute routing differ from convention-based routing?

Convention-based routing defines patterns globally, usually in the Startup.cs file. Attribute routing uses annotations like [Route(“api/products/{id}”)] directly on controllers or actions. It gives more control and clarity at the endpoint level.

12. How would you handle exceptions globally in Web API?

In ASP.NET Core, use UseExceptionHandler() middleware for global exception handling. You can also write custom middleware or use filters like ExceptionFilterAttribute to catch and log errors without repeating code in each controller.

Advanced Level Interview Questions on Web API for Experienced Professionals 

Let’s go through some advanced-level interview questions in Web API and their answers.

13. How do you implement custom middleware in ASP.NET Core Web API?

To create custom middleware, write a class with an Invoke or InvokeAsync method that accepts HttpContext. In the method, you can inspect or modify requests and responses. Then register the middleware in the Startup.cs file using app.UseMiddleware<YourMiddleware>().

14. What is the role of dependency injection in Web API?

Dependency injection (DI) helps you pass required services into controllers or other components. It supports better testing, reduces tight coupling, and follows SOLID principles. In ASP.NET Core, services are added in Program.cs using builder.Services.AddTransient, AddScoped, or AddSingleton.

15. How do you secure APIs for internal use only?

To keep APIs internal, restrict them by IP address using middleware or a reverse proxy like NGINX. You can also use API keys with internal values, or configure access rules in the API gateway if one is in place. Azure and AWS both support private endpoints too.

16. What are the best practices for designing scalable APIs?

Use pagination for large datasets. Keep endpoints resource-focused and consistent. Use async methods to avoid blocking threads. Avoid large payloads and prefer stateless communication. Monitor response times and apply caching wherever possible.

17. How would you use caching in Web API to improve performance?

Use in-memory caching like IMemoryCache for short-term storage or IDistributedCache for distributed systems. Apply [ResponseCache] attributes on actions or configure caching in middleware. Cache responses, not just data, when performance matters.

18. How can you throttle or rate-limit requests in Web API?

Rate limiting prevents abuse. Use third-party packages like AspNetCoreRateLimit or write custom middleware. Limit by IP, endpoint, or user. APIs behind gateways like Azure API Management or AWS API Gateway can configure throttling rules per subscription.

Web API Interview Questions for 5 Years Experienced

• What is your approach to versioning an API in production?
• Can you describe a challenging bug you fixed in an API project?
• How do you handle disagreements in an API design discussion with a teammate?
• How do you test the performance of an API endpoint?

Web API Interview Questions for 10 Years Experienced

• How do you evaluate whether an API should be RESTful or follow another pattern?
• What has been your most complex API integration project?
• How do you mentor junior developers working on API modules?
• What security concerns do you prioritize in enterprise-grade APIs?

Web API Interview Questions for 15 Years Experienced

• How do you design APIs that are resilient to failure?
• Describe a leadership decision you made during a major API migration.
• How do you promote API governance and consistency across teams?
• How do you balance business goals with long-term API architecture?

Web API Scenario Based Interview Questions

Here are the commonly asked Web API scenario based interview questions for experienced professionals. 

19. How would you debug an API that works locally but fails in production?

Start by checking production logs for errors. Compare environment settings – API keys, base URLs, or connection strings often differ. Look at CORS issues or missing headers. Test the API using tools like Postman or curl in the production environment.

20. If an endpoint suddenly starts returning 500 errors, how do you approach the issue?

Check logs for exceptions and stack traces. Look for recent code changes or deployments. I would reproduce the request manually and inspect inputs. If needed, I’d roll back changes and isolate the failing service or database query.

21. How do you expose different response shapes based on user roles?

Use role-based conditionals in your controller. For example, show limited data to regular users and full details to admins. You can create view models tailored to each role and return the appropriate one based on user claims.

22. You need to deprecate an old API version. What steps would you follow?

First, notify consumers via email or headers (Deprecation, Sunset). Provide documentation for the new version. Keep the old version available for a transition period. After the deadline, disable it gradually – returning 410 (Gone) status if needed.

23. How would you handle high latency on a frequently used endpoint?

I would profile the endpoint using tools like Application Insights or ELK Stack. Caching is key—either output caching or data-level caching. I would optimize the database queries, offload work to background jobs, and if needed, add CDN or queueing.

Other Important Interview Questions in Web API

This section covers additional interview questions in Web API that don’t fit into a specific level but are often asked in technical rounds.

ASP.NET Core Web API Interview Questions

Here is a list of ASP.NET Web API interview questions commonly asked during technical interviews.

1. How do you configure dependency injection in ASP.NET Core Web API?
2. What is the difference between IApplicationBuilder and IWebHostBuilder?
3. How do you implement logging in ASP.NET Core Web API?
4. How does the middleware pipeline work in ASP.NET Core?
5. What is the purpose of the appsettings.json file in ASP.NET Core?

Note – .NET Core Web API interview questions often include topics like dependency injection, middleware pipeline, attribute routing, and async programming with controllers.

Web API C# Interview Questions

This section covers Web API interview questions in C# related to routing, controllers, model binding, and error handling.

1. How do you handle model validation in a Web API controller?
2. What is the use of IActionResult in C# Web API?
3. How do you create custom exceptions in a Web API project?
4. What is the role of async/await in Web API methods?
5. How do you serialize and deserialize JSON in Web API?

Classic Web API.NET Interview Questions

Here are DOT NET Web API interview questions frequently asked to test your skills in building and consuming APIs using the .NET framework.

1. How is routing configured in classic ASP.NET Web API?
2. What is the use of HttpResponseMessage in .NET Web API?
3. How do you return different response types in classic Web API?
4. What is the role of Global.asax in ASP.NET Web API?
5. How do you handle cross-cutting concerns in classic Web API?

Also Read - Top 20+ Postman Interview Questions and Answers

Tips to Prepare for Web API Interview 

Preparing for a Web API interview means understanding core concepts, real-world scenarios, and being confident with code examples.

• Review how HTTP methods (GET, POST, PUT, DELETE) work
• Understand status codes and when to use them
• Learn routing and attribute routing differences
• Practice writing clean and RESTful endpoints
• Read about middleware and filters in ASP.NET Core
• Practice handling exceptions in a global way
• Work with model binding and validation examples
• Use tools like Postman to test APIs
• Stay updated with .NET Core features
• Try solving small API problems or bugs on your own

Also Read - Top 50+ REST API Interview Questions and Answers

Wrapping Up

These 25+ Web API interview questions and answers give you a solid understanding of what interviewers expect – whether you are a fresher or experienced professional. They cover real-world scenarios, coding concepts, and practical use cases to help you feel prepared and confident.

Ready to find your next opportunity? Head over to Hirist, the go-to job portal for IT professionals. Find the top Web API jobs in India and apply with ease.

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