What is a Web API?

1. You Already Know APIs. Here's Proof.

Open Zomato. Search "Pizza".

Did Zomato download a full website to your phone? No. Your phone has the app. The app just asked Zomato's server: "Give me pizza places"

Server replied with just this text:

[{"name":"Domino's","rating":4.2,"time":"30 min"}]

That URL is a Web API. That text is JSON.

Your app took that data and drew the screen. Website? Not involved.

2. Why Your Career Depends On This

2026 Job Reality:

• MVC Only: You maintain old company websites. 6-9 LPA. Competing with 10,000 freshers.
• API + MVC: You build backends for mobile apps, React, Angular, microservices. 12-20 LPA. Competing with 1,000 skilled devs.

Every startup, every MNC, every product has an app. Apps need APIs. No API skills = No callback.

3. Build Your First API Right Now - 3 Minutes

We're building the actual API Zomato's app would call.

Step 1: Create Project

VS → New → ASP.NET Core Web API → Name: ZomatoBackend →.NET 8 → Check HTTPS, OpenAPI, Use controllers → Create

Step 2: Add Restaurant Model

Delete WeatherForecast. Add folder Models → Class Restaurant.cs:

public class Restaurant {
    public int Id { get; set; }
    public string Name { get; set; }
    public double Rating { get; set; }
}

Step 3: Add Zomato Controller

Controllers → Add → API Controller Empty → ZomatoController.cs:

[Route("api/[controller]")]
[ApiController]
public class ZomatoController : ControllerBase
{
    [HttpGet("restaurants")]
    public ActionResult> GetRestaurants()
    {
        var list = new List {
            new Restaurant{Id=1, Name="KFC", Rating=4.1},
            new Restaurant{Id=2, Name="Pizza Hut", Rating=4.3}
        };
        return Ok(list);
    }
}

Step 4: Run It

Press F5. Swagger opens. Test GET /api/Zomato/restaurants.

You just did what Zomato's backend team does. You sent data to an app. No HTML. Just JSON.

1. The Interview Theory: What, Why, How

What is a Web API? A URL endpoint that speaks HTTP and exchanges data, usually JSON, between software systems. It has no UI.

Why does it exist? Separation of Concerns. The backend team handles data + rules. Frontend/Mobile teams handle UI. They work parallel. Ship faster. Scale independently.

How does it work? HTTP Request-Response. Client sends Request with Method + URL + Headers + Body. Server sends Response with Status Code + Headers + Body.

2. The #1 Interview Question: "Explain Stateless"

Theory: HTTP is Stateless. The server treats every request as brand new. It has zero memory of your last request. No Session state.

Interviewer asks: "Then how does login work?"

Your Answer: "We don't use server sessions. On login, server gives client a signed JWT token. Client stores it. Every next request, client sends Authorization: Bearer <token> in the header. Server validates the token signature. No memory needed. This is why APIs can scale to millions - any server can handle any request."

This answer gets you the job. "We use Session" gets you rejected.

3. MVC vs API: The CTO View

4. [ApiController] - Your Safety Net

This attribute saves your job. It does 3 things:

1. Auto 400 Validation: Client sends { "rating": "five" } for double rating. Framework returns 400 Bad Request instantly. Your code never runs. No null crashes. No angry users.
2. Binding Inference: Knows complex objects come from [FromBody]. Primitives from query. Less code, fewer bugs.
3. Problem Details Standard: All errors return RFC 7807 JSON. Mobile devs can parse it once and handle all errors. You look professional.

5. Return Types: Senior vs Junior Code

Theory: Your API is a contract. Swagger turns your C# into documentation for other teams.

// JUNIOR: Always 200 OK. Even if restaurant not found. Lying to client.
public Restaurant Get(int id) {...}

// MID-LEVEL: Can return 404. But Swagger shows return type "object".
// Frontend team must guess the shape. They will ping you at midnight.
public IActionResult Get(int id) {...}

// SENIOR: Swagger shows exact Restaurant model + you can return 404.
// Frontend runs one command: nswag. Gets full TypeScript client. Zero meetings.
public ActionResult Get(int id) {
    var r = Find(id);
    return r == null? NotFound() : r;
}