1. Chengyu Sun California State University, Los Angeles
CS4540 Special Topics in Web Development ASP.NET Core: Handling Requests with Controllers
Chengyu Sun
California State University, Los Angeles

2. Controllers in ASP.NET Core MVC
Any class that meets at least one of the two conditions:
Have a name ending in Controller
Inherit from ControllerBase or Controller class
Usually placed in the Controllers folder but can be anywhere in the project

3. A Controller Example public class EmployeesController : Controller {
public IActionResult List()
var employees = employeeService.GetEmployees();
return View( employees ); }

4. What Does An Action Method Return?
An action method can return a view (or part of a view), which renders a response (or part of a response)
An action method can also return a response, which can be a redirect, error, file, JSON, and so on
IActionResult is the interface for all the possible return values of an action method

5. A Quick Look at the ControllerBase Class
The base class for Controller
Properties for accessing various data related to a request
Helper methods for creating various responses, e.g. Ok, NotFound, File, Redirect
Usually used as the base class for web API controllers

6. A Quick Look at the Controller Class
Inherits from ControllerBase
Adds support for views
Type
Name
Usage
Property
ViewBag, ViewData
For passing additional data to views
TempData
Single-use session data (usually used for redirect)
Method
View, PartialView, ViewComponent, Json
For creating ViewResult or JsonResult
OnActionExecuted, OnActionExecuting, OnActionExecutionAsync, Dispose
Controller life cycle hooks

7. ViewData ViewData is a Dictionary<string,object> In controller:
ViewData["employees"] = employees;
Require casting
In view:
@foreach (var employee in (List<Employee>) ViewData["employees"]) {
<tr>
</tr> }

8. ViewBag ViewBag is an ExpandoObject (i.e. expandable object)
Supports dynamic properties whose types are determined at runtime, i.e. no casting necessary
A wrapper of ViewData
Example:
ViewBag.employees = employees;

9. View Model vs. ViewData vs. ViewBag
Type declared in view
Type can be determined at compilation time, thus allows code suggestion/completion in IDEs
ViewData
Dictionary<string,object>
Require casting from object to the real type of the data
ViewBag
ExpandoObject
Property types are determined at runtime – no casting required
Why do we have ViewData when ViewBag seems better??

10. Example: View an Employee
Route:
"{controller=Home}/{action=Index}/{id?}"
URL:
Employees / View / 1
EmployeesController:
IActionResult View( int id )

11. Routing in ASP.NET Core Map URLs of requests to controller actions
Conventional routing is based on routes specified for the MVC middleware (i.e. each route establishes a convention)
Attribute routing specifies mapping on a controller or action method, e.g.
[Route("Employees/List")]
public IActionResult List() { … }

12. A Route in Conventional Routing
routes.MapRoute(
name: "default",
template: "{controller}/{action}/{id?}",
defaults: new {
controller = "Home",
action = "Index"
});
A route in conventional routing must have a controller and an action

13. Route Template Route parameters
"{controller=Home} / {action=Index} / {id?}"
No leading /
Route parameters must be separated by literal values
controller and action are reserved names
Default value and optional parameter

14. Example: View Two Employees
Use URL like /Employees/Two/1/2
Use URL like /Employees/Two/1-2
Routes order is important – router will try each route in order and stop at the first match

15. Example: Wiki URLs
Use a catch-all route parameter, e.g. "wiki/{*path}" or "wiki/{**path}"
There's no difference between * and ** when using the parameter in an action method

16. Attribute Routing Examples
[Route("Home/About")]
public IActionResult About() { … }
[Route("Help")]
[Route("Home/About")]
public IActionResult About() { … }
[HttpGet]
[Route("Employees/View/{id}")]
public IActionResult View(int id) { … }
[HttpGet("Employees/View/{id}")]
public IActionResult View(int id) { … }

17. Route Constraints
Route constraints can be used to restrict how the route parameters are match, e.g.
[Route("users/{id:int}")]
public User GetUserById(int id) { ... }
[Route("users/{name}")]
public User GetUserByName(string name) { ... }

18. Mixed Routing
A web application can use both conventional and attribute routing
Attribute routing takes precedence

19. Example: Add an Employee
Routing based on HTTP request methods
Binding
Simple type
Complex type
RedirectToAction()
More refactoring-friendly
Easier to specify route parameters

20. Binding
Map the data in a request to the parameters of an action method
Request
Add(
string name,
DateTime dateHired )
Add(
Employee employee )

21. Binding Sources Route parameters Query parameters Request body
Posted form fields
Uploaded files
Objects (e.g. JSON or XML)
Request headers
Defaults

22. Choosing Binding Source Using Attribute
[FromRoute]
[FromQuery]
[FromForm]
[FromBody]
[FromHeader]

23. Binding Collections List<int> numbers numbers=100&numbers=200
Dictionary<string,string> courses
courses[4022]=Databases&
courses[4050]=DotnetCore
courses[0].Key=4022&courses[0].Value=Databases&
courses[1].Key=4050&courses[1].Value=DotnetCore

24. Models in MVC Controller Database View MVC Application HTTP request
Domain
Model
Binding
Model
View
Model
Database
HTTP
response
View
MVC Application

25. About Model Class
It's convenient to use the same model class for the different models, but some sometimes it may be necessary to create separate classes, e.g. to prevent over-posting attack

26. Readings ASP.NET Core in Action: Chapter 4, 5, 6
Routing in ASP.NET Core
Model Binding in ASP.NET Core