Building REST APIs — Complete Guide

Building REST APIs — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of ASP.NET Core Tutorial on Toolliyo Academy.

8 min read Updated 6/25/2026

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ASP.NET Core Tutorial (ShopNest) · Lesson 41 of 100

Building REST APIs

Beginner ✓ → Intermediate ✓ → Advanced → Professional

Advanced · 3 — Production skills · ~18 min read · Module 5: Web API & Security

Introduction

This is advanced material: Building REST APIs. It is what teams use on live products. Read the example carefully and try changing one line at a time to see what happens. A REST API returns JSON over HTTP. GET reads data, POST creates, PUT updates, DELETE removes. Mobile apps and React frontends call these endpoints. ShopNest needs an API so the storefront can load products without full page reloads.

APIs are how your backend talks to React, Angular, or mobile apps. Get routing and JSON responses solid here.

When will you use this?

Use Web API when a website, mobile app, or React frontend needs JSON from your server.

Mobile apps and React frontends call your ASP.NET Core API over HTTP with JSON.
JWT tokens prove who the user is on every protected API request.

Real-world: Toolliyo-style learning platform

The EdTech / LMS team building Toolliyo-style learning platform uses Building REST APIs to expose /api/products for mobile app and React frontend. students and instructors never see the C# code — they just get a fast, reliable course API and lesson progress tracking.

Production-style code

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    [HttpGet]
    public IActionResult GetAll() =>
        Ok(new[] { new { id = 1, name = "Keyboard", price = 2499 } });

    [HttpGet("{id:int}")]
    public IActionResult GetById(int id) =>
        Ok(new { id, name = "Keyboard", price = 2499 });
}

What happens in production: In Toolliyo-style learning platform, getting Building REST APIs right means students and instructors trust the course API and lesson progress tracking every day.

Lesson example (start here)

Copy this smaller example first. Once it works, compare it with the real-world code above.

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    [HttpGet]
    public IActionResult GetAll() =>
        Ok(new[] { new { id = 1, name = "Mouse", price = 499 } });

    [HttpGet("{id:int}")]
    public IActionResult GetById(int id) =>
        Ok(new { id, name = "Mouse", price = 499 });
}

Line-by-line walkthrough

Code	What it means
`[ApiController]`	Attribute — tells ASP.NET Core how to route or secure this class/method.
`[Route("api/[controller]")]`	Attribute — tells ASP.NET Core how to route or secure this class/method.
`public class ProductsController : ControllerBase`	Controller class — handles HTTP requests and returns views or JSON.
`{`	Part of the Building REST APIs example — read it together with the lines before and after.
`[HttpGet]`	Attribute — tells ASP.NET Core how to route or secure this class/method.
`public IActionResult GetAll() =>`	Return type — can be a view, redirect, JSON, or error response.
`Ok(new[] { new { id = 1, name = "Mouse", price = 499 } });`	Part of the Building REST APIs example — read it together with the lines before and after.
`[HttpGet("{id:int}")]`	Attribute — tells ASP.NET Core how to route or secure this class/method.
`public IActionResult GetById(int id) =>`	Return type — can be a view, redirect, JSON, or error response.
`Ok(new { id, name = "Mouse", price = 499 });`	Part of the Building REST APIs example — read it together with the lines before and after.
`}`	Closes a block started by { above.

How it works (big picture)

[ApiController] enables automatic validation.
Route template api/products maps to this class.
Ok() returns 200 with JSON body.

Do this on your computer

dotnet new webapi -n ShopNest.Api
Add a ProductsController with HttpGet actions.
Run and open /swagger.
Try GET /api/products in the browser.
Read the real-world section and name which part of the app uses this topic.
Run the example locally with dotnet run and confirm the same behavior.
Change one value in the example (route, text, or connection string) and predict what will happen before you save.

Experiments — try changing this

Change a string or route in the example and save — watch the browser or Swagger response update.
Break the code on purpose (remove a semicolon), read the error message, then fix it.
Change the URL path and update the browser address to match.
Use dotnet watch run while editing Building REST APIs — the app restarts on save.

Remember

Web API = JSON + HTTP verbs. Use ControllerBase for APIs. Swagger helps you test endpoints quickly.

Common questions

MVC controller vs API controller?

MVC returns HTML views; API returns JSON with ControllerBase.

How long should I spend on Building REST APIs?

Until you can explain it in your own words and run the example without looking at the answer. Beginners often need 30–60 minutes per new concept; setup lessons may take one afternoon.

What if I get stuck on Building REST APIs?

Re-read the line-by-line walkthrough, check the terminal for red errors, and compare your code character-by-character with the example. Search the exact error text — someone else had it too.

Where is Building REST APIs used in real jobs?

See the real-world section above — the same pattern appears in LMS, banking, e-commerce, and SaaS backends. Interviewers ask you to explain it using one concrete example.