How do you create an API in ASP.NET Core?

Utpal Vishwas · 2025-05-20T22:21:29.627Z

Use [ApiController] and [Route(\"api/[controller]\")] attributes on controllers. Use [HttpGet] , [HttpPost] , etc., to define actions.

Use [ApiController] and [Route("api/[controller]")] attributes on controllers.
Use [HttpGet], [HttpPost], etc., to define actions.

Creating an API in ASP.NET Core is straightforward thanks to the built-in Web API template. Here's a step-by-step guide.

Step 1: Create a New ASP.NET Core Web API Project

Using the .NET CLI:

# Create a new Web API project
dotnet new webapi -n ProductApi

# Navigate to the project directory
cd ProductApi

Or in Visual Studio:

Create a new project.
Select ASP.NET Core Web API.
Enter the project name.
Click Create.

Step 2: Examine the Project Structure

A typical project structure looks like:

ProductApi
│
├── Controllers
├── Program.cs
├── appsettings.json
├── Properties
└── ProductApi.csproj

Step 3: Create a Model

Create a Models folder and add a Product class.

namespace ProductApi.Models;

public class Product
{
    // Product identifier
    public int Id { get; set; }

    // Product name
    public string Name { get; set; } = string.Empty;

    // Product price
    public decimal Price { get; set; }
}

Step 4: Create a Controller

Inside the Controllers folder, create ProductsController.cs.

using Microsoft.AspNetCore.Mvc;
using ProductApi.Models;

namespace ProductApi.Controllers;

[ApiController]
[Route("api/[controller]")]
public class ProductsController : ControllerBase
{
    // Sample in-memory data
    private static readonly List<Product> Products =
    [
        new Product
        {
            Id = 1,
            Name = "Laptop",
            Price = 50000
        },
        new Product
        {
            Id = 2,
            Name = "Mouse",
            Price = 1000
        }
    ];

    // GET: api/products
    [HttpGet]
    public IActionResult GetAll()
    {
        return Ok(Products);
    }
}

Step 5: Configure Services

In .NET 6+ (Program.cs):

var builder = WebApplication.CreateBuilder(args);

// Add controller services
builder.Services.AddControllers();

// Add Swagger services
builder.Services.AddEndpointsApiExplorer();
builder.Services.AddSwaggerGen();

var app = builder.Build();

// Enable Swagger
app.UseSwagger();
app.UseSwaggerUI();

// Map controllers
app.MapControllers();

app.Run();

Step 6: Run the API

dotnet run

Example output:

Now listening on:
https://localhost:7001
http://localhost:5001

Step 7: Test the API

Open:

https://localhost:7001/swagger

Swagger UI allows you to test endpoints directly from the browser.

Example response from GET /api/products:

[
  {
    "id": 1,
    "name": "Laptop",
    "price": 50000
  },
  {
    "id": 2,
    "name": "Mouse",
    "price": 1000
  }
]

Step 8: Add a POST Endpoint

[HttpPost]
public IActionResult Create(Product product)
{
    // Generate a new Id
    product.Id = Products.Max(p => p.Id) + 1;

    // Add product to collection
    Products.Add(product);

    return CreatedAtAction(
        nameof(GetById),
        new { id = product.Id },
        product);
}

Add a GET by Id endpoint:

[HttpGet("{id}")]
public IActionResult GetById(int id)
{
    // Find product by Id
    var product = Products.FirstOrDefault(p => p.Id == id);

    if (product == null)
        return NotFound();

    return Ok(product);
}

Step 9: Add PUT Endpoint

[HttpPut("{id}")]
public IActionResult Update(int id, Product updatedProduct)
{
    // Find existing product
    var product = Products.FirstOrDefault(p => p.Id == id);

    if (product == null)
        return NotFound();

    // Update values
    product.Name = updatedProduct.Name;
    product.Price = updatedProduct.Price;

    return NoContent();
}

Step 10: Add DELETE Endpoint

[HttpDelete("{id}")]
public IActionResult Delete(int id)
{
    // Find product
    var product = Products.FirstOrDefault(p => p.Id == id);

    if (product == null)
        return NotFound();

    // Remove product
    Products.Remove(product);

    return NoContent();
}

Complete CRUD Endpoints

HTTP Method	Endpoint	Description
GET	`/api/products`	Get all products
GET	`/api/products/{id}`	Get product by ID
POST	`/api/products`	Create product
PUT	`/api/products/{id}`	Update product
DELETE	`/api/products/{id}`	Delete product

Next Steps for Production Applications

After creating a basic API, you'll typically add:

Entity Framework Core for database access.
Dependency Injection for services and repositories.
Validation using Data Annotations or FluentValidation.
Authentication & Authorization using JWT tokens.
Logging with the built-in logging framework.
Exception Handling Middleware.
API Versioning.
Unit Testing with xUnit and Moq.
Docker Support.
CI/CD Pipelines using GitHub Actions or Azure DevOps.

This is the standard approach for building RESTful APIs in modern ASP.NET Core applications.

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