Build Your First ASP.NET Core Web API Introduction

Modern applications depend heavily on APIs—from mobile apps to single-page web applications and microservices.
ASP.NET Core makes building fast, secure, and scalable Web APIs simple and developer-friendly.

In this article, you’ll build your first ASP.NET Core Web API from scratch, understand how it works, and learn best practices used in real-world projects.

By the end of this post, you will:

• Create a REST API using ASP.NET Core

• Understand controllers, routes, and HTTP methods

• Test APIs using Swagger

• Follow clean API design principles

What Is an ASP.NET Core Web API?

An ASP.NET Core Web API is a backend service that:

• Exposes endpoints over HTTP

• Uses JSON as data format

• Follows REST principles

• Can be consumed by frontend apps, mobile apps, or other services

Typical API Use Cases

• User authentication

• CRUD operations

• Mobile app backend

• Microservices communication

Step 1: Create a New Web API Project

Run the following command:

dotnet new webapi -n FirstWebApi
cd FirstWebApi
dotnet run

Once started, open your browser and navigate to:

https://localhost:5001/swagger

🎉 Congratulations! Your API is already running.

Step 2: Understanding Project Structure

FirstWebApi/
│
├── Controllers/
│   └── WeatherForecastController.cs
│
├── Program.cs
├── appsettings.json
└── FirstWebApi.csproj

Key Files Explained

| File             | Purpose                  |
| ---------------- | ------------------------ |
| Program.cs       | App startup & middleware |
| Controllers      | API endpoints            |
| appsettings.json | Configuration            |
| Swagger          | API documentation        |

Step 3: Understanding Controllers & Routing

Sample Controller

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    [HttpGet]
    public IActionResult GetAll()
    {
        return Ok(new[] { "Laptop", "Phone", "Tablet" });
    }
}