ASP.NET Core Basics

# CHAPTER 26

ASP.NET Core Basics #

1. Introduction #

2. Learning Objectives #

• Understand what ASP.NET Core is.

• Create an ASP.NET Core Web API project.

• Understand the MVC (Model-View-Controller) architecture.

• Write a Controller to handle HTTP requests.

• Understand Routing.

3. What is ASP.NET Core? #

There are two main ways to use it:

1. 1. Web API: Returns JSON data. Used as the backend for React/Angular apps or Mobile apps.

1. 2. MVC / Razor Pages: Returns full HTML pages directly to the browser.

4. Creating a Web API Project #

dotnet new webapi -n MyBackendApp
cd MyBackendApp
dotnet run

The application will start a local web server (e.g., http://localhost:5000).

5. The MVC Architecture #

• Model: Represents the data (e.g., a C# User class).

• View: The User Interface (HTML/CSS). *Not used in Web APIs.*

• Controller: The brain. It receives the HTTP request, grabs the Model data, and returns the response.

6. Writing a Controller #

using Microsoft.AspNetCore.Mvc;
using System.Collections.Generic;

namespace MyBackendApp.Controllers
{
    // Routing: This controller answers requests sent to "http://localhost:5000/api/users"
    [ApiController]
    [Route("api/[controller]")]
    public class UsersController : ControllerBase
    {
        // Answers a standard GET request
        [HttpGet]
        public IActionResult GetAllUsers()
        {
            var users = new List<string> { "Alice", "Bob", "Charlie" };
            
            // Automatically converts the C# List into a JSON response!
            return Ok(users); 
        }

        // Answers a GET request with an ID parameter in the URL (e.g., /api/users/5)
        [HttpGet("{id}")]
        public IActionResult GetUserById(int id)
        {
            return Ok($"You requested user ID: {id}");
        }
    }
}

7. Routing Explained #

• [Route("api/[controller]")] tells ASP.NET to take the class name (UsersController), drop the word "Controller", and use api/Users as the base URL.

• [HttpGet("{id}")] adds to the route. If the browser asks for /api/users/99, ASP.NET finds this method and passes 99 into the id parameter automatically!

8. Modern C# Minimal APIs #

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

// A Minimal API Endpoint
app.MapGet("/api/hello", () => "Hello from Minimal API!");

app.Run();

9. Common Mistakes #

• Forgetting [ApiController]: If you forget this attribute on a controller class, automatic JSON validation and routing behaviors will fail.

• Port Conflicts: If dotnet run fails, your port (e.g., 5000) might be in use by another application.

10. Best Practices #

• Keep Controllers thin. Controllers should only handle receiving the request and returning the response. Complex business logic and database code should be moved to separate "Service" classes.

11. Exercises #

1. 1. Create a dotnet new webapi project.

1. 2. Inside Program.cs, create a Minimal API endpoint app.MapGet("/api/time", () => DateTime.Now.ToString());.

1. 3. Run the project and visit the URL in your browser to see the live server time.

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the MVC architecture as it applies to ASP.NET Core.

• Q: What is the difference between an ASP.NET Web API and an ASP.NET MVC/Razor Pages application?

• Q: Explain how Dependency Injection works in the context of ASP.NET Core controllers.

14. Summary #

15. Next Chapter Recommendation #