Day 3 - What is Controllers

Controllers in ASP.NET MVC Applications

The ASP.NET MVC framework maps URLs to classes that are referred to as controllers. Controllers process incoming requests, handle user input and interactions, and execute appropriate application logic. A controller class typically calls a separate view component to generate the HTML markup for the request.

The base class for all controllers is the ControllerBase class, which provides general MVC handling. The Controller class inherits from ControllerBase and is the default implement of a controller. The Controller class is responsible for the following processing stages:

Locating the appropriate action method to call and validating that it can be called.
Getting the values to use as the action method's arguments.
Handling all errors that might occur during the execution of the action method.
Providing the default WebFormViewEngine class for rendering ASP.NET page types (views).

[HandleError]
public class HomeController : Controller
{
    public ActionResult Index()
    {
        ViewData["Message"] = "Welcome to ASP.NET MVC!";

        return View();
    }

    public ActionResult About()
    {
        return View();
    }
}

Action Methods





In ASP.NET applications that do not use the MVC framework, user interaction is organized around pages, and around raising and handling events from the page and from controls on the page. In contrast, user interaction with ASP.NET MVC applications is organized around controllers and action methods. The controller defines action methods. Controllers can include as many action methods as needed.

Action methods typically have a one-to-one mapping with user interactions. Examples of user interactions include entering a URL into the browser, clicking a link, and submitting a form. Each of these user interactions causes a request to be sent to the server. In each case, the URL of the request includes information that the MVC framework uses to invoke an action method.

When a user enters a URL into the browser, the MVC application uses routing rules that are defined in the Global.asax file to parse the URL and to determine the path of the controller. The controller then determines the appropriate action method to handle the request. By default, the URL of a request is treated as a sub-path that includes the controller name followed by the action name. For example, if a user enters the URL http://contoso.com/MyWebSite/Products/Categories, the sub-path is /Products/Categories. The default routing rule treats "Products" as the prefix name of the controller, which must end with "Controller" (such as ProductsController). It treats "Categories" as the name of the action. Therefore, the routing rule invokes the Categories method of the Products controller in order to process the request. If the URL ends with /Products/Detail/5, the default routing rule treats "Detail" as the name of the action, and the Detail method of the Products controller is invoked to process the request. By default, the value "5" in the URL will be passed to the Detailmethod as a parameter.

The following example shows a controller class that has a HelloWorld action method.
public class MyController : Controller
{
    public ActionResult HelloWorld()
    {
        ViewData["Message"] = "Hello World!";
        return View();
    }
}