ASP.NET/IntroductorySessions/Session02/Notes/02_Notes.md

Session 2: Middleware, Routing, and Controllers in ASP.NET Core MVC

📝 Overview

In this session, we’ll cover the following concepts:

• Middleware and the ASP.NET Core request pipeline
• Routing structure: default, attribute-based, constraints, SEO-friendly
• Creating and structuring controllers
• Handling HTTP methods
• Dependency Injection in controllers
• Model binding and action parameters

📚 Topics Covered

✅ Middleware and Request Pipeline

Visual understanding of the middleware flow and pipeline order
🖼️ See diagrams in original session notes

🔗 ASP.NET Core Middleware Overview

✅ Routing in ASP.NET Core

Learn how incoming requests map to controller actions through routing.
🔗 ASP.NET Core Routing Docs

✅ Controllers

Understand how to structure controllers and handle requests with actions.
🔗 ASP.NET Core Controllers

📌 Notes

Collected from various sources including Microsoft Docs and ChatGPT

---

📍 Part 0: Middleware and Request Pipeline

🖼️ Diagrams:

• !
• !
• !

---

📍 Part 1: Routing in ASP.NET Core MVC

Routing maps incoming HTTP requests to controller actions.

✅ Basic Routing Structure

Routing is configured in Program.cs via app.MapControllerRoute.

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}"
);

- `{controller=Home}`: default controller

- `{action=Index}`: default action

- `{id?}`: optional URL parameter


---

### ✅ Attribute Routing

Define routes directly in the controller using attributes:

```csharp
[Route("products")]
public class ProductsController : Controller
{
    [HttpGet("all")]
    public IActionResult GetAllProducts() { /*...*/ }

    [HttpGet("{id}")]
    public IActionResult GetProductById(int id) { /*...*/ }
}

---

✅ Route Parameters & Constraints

You can enforce patterns and types using constraints:

app.MapControllerRoute(
    name: "custom",
    pattern: "{controller=Products}/{action=List}/{id:int:min(1)}"
);

• int:min(1) means id must be an integer ≥ 1
• Other constraints: bool, datetime, guid, minlength(x), etc.

---

✅ Per-Controller Default Routes

You can assign a default action to a specific controller:

app.MapControllerRoute(
    name: "gallery",
    pattern: "Gallery/{action=Main}/{id?}",
    defaults: new { controller = "Gallery" }
);

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}"
);

---

✅ SEO-Friendly URLs

Use descriptive URLs for readability and SEO:

app.MapControllerRoute(
    name: "productDetail",
    pattern: "products/details/{id:int}/{name}"
);

✅ Example output: /products/details/10/laptop

---

📍 Part 2: Controllers in ASP.NET Core MVC

Controllers handle requests and return responses, acting as a bridge between models and views.

✅ Creating a Basic Controller

public class HomeController : Controller
{
    public IActionResult Index()
    {
        return View();
    }
}

---

✅ Handling HTTP Methods

Use attributes to define the HTTP method an action should handle:

[HttpPost]
public IActionResult CreateProduct(Product product)
{
    return RedirectToAction("Index");
}

Other attributes include:

• [HttpGet]
• [HttpPut]
• [HttpDelete]

---

✅ Dependency Injection in Controllers

Inject services like repositories into controllers for better separation of concerns:

public class ProductsController : Controller
{
    private readonly IProductRepository _repository;

    public ProductsController(IProductRepository repository)
    {
        _repository = repository;
    }
}

---

✅ Action Parameters and Model Binding

Parameters are bound automatically from the URL, query string, or form body:

public IActionResult EditProduct(int id, string name)
{
    // id and name are bound from query or route
}

For complex types, ASP.NET Core binds data from the request body (e.g., forms or JSON).

---

🧪 Practice

• Define multiple routes with custom constraints
• Build an SEO-friendly route pattern
• Create controller with basic actions (Index, Details, Create)
• Add attribute-based routes to actions
• Use [HttpGet], [HttpPost] on appropriate methods
• Inject a service into a controller via constructor
• Bind parameters from query and route

🙏 Acknowledgments

Sources:

• Microsoft Docs
• ChatGPT 2025 sessions