ASP.NET MVC Controller Overview (C#)

by Stephen Walther

In this tutorial, Stephen Walther introduces you to ASP.NET MVC controllers. You learn how to create new controllers and return different types of action results.

This tutorial explores the topic of ASP.NET MVC controllers, controller actions, and action results. After you complete this tutorial, you will understand how controllers are used to control the way a visitor interacts with an ASP.NET MVC website.

Understanding Controllers

MVC controllers are responsible for responding to requests made against an ASP.NET MVC website. Each browser request is mapped to a particular controller. For example, imagine that you enter the following URL into the address bar of your browser:

http://localhost/Product/Index/3

In this case, a controller named ProductController is invoked. The ProductController is responsible for generating the response to the browser request. For example, the controller might return a particular view back to the browser or the controller might redirect the user to another controller.

Listing 1 contains a simple controller named ProductController.

Listing1 - Controllers\ProductController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Ajax;

namespace MvcApplication1.Controllers
{
    public class ProductController : Controller
    {
        //
        // GET: /Products/

        public ActionResult Index()
        {
            // Add action logic here
            return View();
        }
    }
}

As you can see from Listing 1, a controller is just a class (a Visual Basic .NET or C# class). A controller is a class that derives from the base System.Web.Mvc.Controller class. Because a controller inherits from this base class, a controller inherits several useful methods for free (We discuss these methods in a moment).

Understanding Controller Actions

A controller exposes controller actions. An action is a method on a controller that gets called when you enter a particular URL in your browser address bar. For example, imagine that you make a request for the following URL:

http://localhost/Product/Index/3

In this case, the Index() method is called on the ProductController class. The Index() method is an example of a controller action.

A controller action must be a public method of a controller class. C# methods, by default, are private methods. Realize that any public method that you add to a controller class is exposed as a controller action automatically (You must be careful about this since a controller action can be invoked by anyone in the universe simply by typing the right URL into a browser address bar).

There are some additional requirements that must be satisfied by a controller action. A method used as a controller action cannot be overloaded. Furthermore, a controller action cannot be a static method. Other than that, you can use just about any method as a controller action.

Understanding Action Results

A controller action returns something called an action result. An action result is what a controller action returns in response to a browser request.

The ASP.NET MVC framework supports several types of action results including:

1. ViewResult - Represents HTML and markup.
2. EmptyResult - Represents no result.
3. RedirectResult - Represents a redirection to a new URL.
4. JsonResult - Represents a JavaScript Object Notation result that can be used in an AJAX application.
5. JavaScriptResult - Represents a JavaScript script.
6. ContentResult - Represents a text result.
7. FileContentResult - Represents a downloadable file (with the binary content).
8. FilePathResult - Represents a downloadable file (with a path).
9. FileStreamResult - Represents a downloadable file (with a file stream).

All of these action results inherit from the base ActionResult class.

In most cases, a controller action returns a ViewResult. For example, the Index controller action in Listing 2 returns a ViewResult.

Listing 2 - Controllers\BookController.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Web.Mvc.Ajax;

namespace MvcApplication1.Controllers
{
    public class BookController : Controller
    {
        public ActionResult Index()
        {
            // Add action logic here
            return View();
        }
    }
}

When an action returns a ViewResult, HTML is returned to the browser. The Index() method in Listing 2 returns a view named Index to the browser.

Notice that the Index() action in Listing 2 does not return a ViewResult(). Instead, the View() method of the Controller base class is called. Normally, you do not return an action result directly. Instead, you call one of the following methods of the Controller base class:

1. View - Returns a ViewResult action result.
2. Redirect - Returns a RedirectResult action result.
3. RedirectToAction - Returns a RedirectToRouteResult action result.
4. RedirectToRoute - Returns a RedirectToRouteResult action result.
5. Json - Returns a JsonResult action result.
6. JavaScriptResult - Returns a JavaScriptResult.
7. Content - Returns a ContentResult action result.
8. File - Returns a FileContentResult, FilePathResult, or FileStreamResult depending on the parameters passed to the method.

So, if you want to return a View to the browser, you call the View() method. If you want to redirect the user from one controller action to another, you call the RedirectToAction() method. For example, the Details() action in Listing 3 either displays a view or redirects the user to the Index() action depending on whether the Id parameter has a value.

Listing 3 - CustomerController.cs

using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    public class CustomerController : Controller
    {
        public ActionResult Details(int? id)
        {
            if (!id.HasValue)
                return RedirectToAction("Index");

            return View();
        }

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

    }
}

The ContentResult action result is special. You can use the ContentResult action result to return an action result as plain text. For example, the Index() method in Listing 4 returns a message as plain text and not as HTML.

Listing 4 - Controllers\StatusController.cs

using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    public class StatusController : Controller
    {
        public ActionResult Index()
        {
            return Content("Hello World!");
        }
    }
}

When the StatusController.Index() action is invoked, a view is not returned. Instead, the raw text "Hello World!" is returned to the browser.

If a controller action returns a result that is not an action result - for example, a date or an integer - then the result is wrapped in a ContentResult automatically. For example, when the Index() action of the WorkController in Listing 5 is invoked, the date is returned as a ContentResult automatically.

Listing 5 - WorkController.cs

using System;
using System.Web.Mvc;

namespace MvcApplication1.Controllers
{
    public class WorkController : Controller
    {
        public DateTime Index()
        {
            return DateTime.Now;
        }
    }
}

The Index() action in Listing 5 returns a DateTime object. The ASP.NET MVC framework converts the DateTime object to a string and wraps the DateTime value in a ContentResult automatically. The browser receives the date and time as plain text.

Summary

The purpose of this tutorial was to introduce you to the concepts of ASP.NET MVC controllers, controller actions, and controller action results. In the first section, you learned how to add new controllers to an ASP.NET MVC project. Next, you learned how public methods of a controller are exposed to the universe as controller actions. Finally, we discussed the different types of action results that can be returned from a controller action. In particular, we discussed how to return a ViewResult, RedirectToActionResult, and ContentResult from a controller action.