ASP.NET Core Web API

What is a Web API?

A Web API allows external clients (like web apps, mobile apps, or other services) to interact with your application via HTTP. Common use cases include:

• CRUD operations (Create, Read, Update, Delete)
• Returning data in JSON or XML formats
• Handling client-server communication in a decoupled manner

Creating a Web API Project

Use the CLI or Visual Studio template:

// CLI
dotnet new webapi -n MyApiApp

By default, it includes:

• Program.cs and Startup.cs (in older versions)
• Controllers/WeatherForecastController.cs
• Support for Swagger UI

Understanding Controllers

Controllers are C# classes that handle incoming HTTP requests.

using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("api/[controller]")]
public class HelloController : ControllerBase
{
    [HttpGet]
    public string Get() => "Hello from API!";
}

This responds to GET /api/hello with a message.

Routing and HTTP Verbs

• [HttpGet] – retrieve data
• [HttpPost] – submit new data
• [HttpPut] – update data
• [HttpDelete] – delete data

Routing is based on attributes and conventions.

Returning JSON Responses

ASP.NET Core automatically serializes objects to JSON by default:

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
}

[HttpGet]
public IActionResult GetProduct()
{
    var product = new Product { Id = 1, Name = "Laptop" };
    return Ok(product); // Returns JSON
}

Running and Testing the API

• Run using dotnet run or F5 in Visual Studio
• Visit Swagger UI at https://localhost:5001/swagger
• Use tools like Postman or curl for manual testing

ASP.NET Core Web API offers a clean and scalable way to expose services over HTTP. In the next module, you’ll learn how to handle parameter binding, action return types, and content negotiation in your APIs.