How to Write a REST API in C# in 30 Minutes: A Step-by-Step Guide

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Introduction: REST API in C# is Simple

Creating a modern REST API in C# using ASP.NET Core is one of the most in-demand tasks in backend development. Many beginner developers think that writing a full-fledged API requires weeks of preparation. In reality, using the right tools, you can create a working REST API in C# in literally 30 minutes.

In this article, we will practically break down how to write a REST API in C# from scratch. We will create a simple microservice for managing a task list (Todo List) using the built-in capabilities of ASP.NET Core. You will learn how to set up routing, implement CRUD operations, and test the API via Swagger UI.

1. Project Setup in 5 Minutes

First, make sure you have .NET SDK 6.0 or newer installed. If not, download it from the official Microsoft website. Open a terminal and run the following commands:

dotnet new webapi -n TodoApicd TodoApicode .

These commands will create a new Web API project and open it in Visual Studio Code. The project structure includes several important files:

Program.cs — entry point and service configuration
Controllers/ — folder for API controllers
appsettings.json — configuration file

By default, the project already contains an example controller WeatherForecastController. We will delete it and create our own.

Important: Make sure the Program.cs file contains the lines builder.Services.AddControllers(); and app.MapControllers();. This is the standard configuration for a REST API in C#.

2. Creating the Data Model and Controller (10 Minutes)

Let's start by creating a data model. In the project root folder, create a Models folder and add a TodoItem.cs file to it:

public class TodoItem{    public int Id { get; set; }    public string Title { get; set; } = string.Empty;    public bool IsComplete { get; set; }    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;}

Now let's create the controller. In the Controllers folder, add a TodoController.cs file. This is the heart of our REST API in C#:

[ApiController][Route("api/[controller]")]public class TodoController : ControllerBase{    private static List<TodoItem> _todos = new();    private static int _nextId = 1;

    [HttpGet]    public IActionResult GetAll()    {        return Ok(_todos);    }

    [HttpGet("{id}")]    public IActionResult GetById(int id)    {        var todo = _todos.FirstOrDefault(t => t.Id == id);        if (todo == null) return NotFound();        return Ok(todo);    }

    [HttpPost]    public IActionResult Create(TodoItem item)    {        item.Id = _nextId++;        _todos.Add(item);        return CreatedAtAction(nameof(GetById), new { id = item.Id }, item);    }

    [HttpPut("{id}")]    public IActionResult Update(int id, TodoItem item)    {        var existing = _todos.FirstOrDefault(t => t.Id == id);        if (existing == null) return NotFound();                existing.Title = item.Title;        existing.IsComplete = item.IsComplete;        return NoContent();    }

    [HttpDelete("{id}")]    public IActionResult Delete(int id)    {        var todo = _todos.FirstOrDefault(t => t.Id == id);        if (todo == null) return NotFound();                _todos.Remove(todo);        return NoContent();    }}

Pay attention to the attributes: