Adding a Controller

MVC stands for model-view-controller.  MVC is a pattern for developing applications that are well architected, testable  and easy to maintain. MVC-based applications contain:

• Models: Classes that represent the data of the application  and that use validation logic to enforce business rules for that data.
• Views: Template files that your application uses to dynamically  generate HTML responses.
• Controllers: Classes that handle incoming browser requests,  retrieve model data, and then specify view templates that return a response  to the browser.

We'll be covering all these concepts in this tutorial series and show you how  to use them to build an application.

Let's begin by creating a controller class. In Solution  Explorer, right-click the Controllers folder  and then click Add,  then Scaffold.

In the Add Scaffold dialog box, click MVC 5  Controller - Empty, and then click Add.

Name your new controller "HelloWorldController" and click Add.

Notice in Solution  Explorer that a new file  has been created named HelloWorldController.cs.  The file is open in the IDE.

Replace the contents of the file with the following code.

using System.Web;
using System.Web.Mvc; 
 
namespace MvcMovie.Controllers 
{ 
    public class HelloWorldController : Controller 
    { 
        // 
        // GET: /HelloWorld/ 
 
        public string Index() 
        { 
            return "This is my <b>default</b> action..."; 
        } 
 
        // 
        // GET: /HelloWorld/Welcome/ 
 
        public string Welcome() 
        { 
            return "This is the Welcome action method..."; 
        } 
    } 
}

The controller methods will return a string of HTML as an example. The controller  is named HelloWorldController and  the first method above is named Index.  Let’s invoke it from a browser. Run the application (press F5 or Ctrl+F5). In the  browser, append "HelloWorld" to the path in the address bar. (For example,  in the illustration below, it's http://localhost:1234/HelloWorld.)  The page in the browser will look like the following screenshot. In the method above,  the code returned a string directly. You told the system to just return some HTML,  and it did! 

ASP.NET MVC invokes different controller classes (and different action methods within  them) depending on the incoming URL. The default URL routing logic used by ASP.NET  MVC uses a format like this to determine what code to invoke:

/[Controller]/[ActionName]/[Parameters]

You set the format for routing in the App_Start/RouteConfig.cs  file.

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
    );
}

When you run the application and don't supply any URL segments, it defaults to  the "Home" controller and the "Index" action method specified in the defaults  section of the code above. 

The first part of the URL determines the controller class to execute. So /HelloWorld maps  to the  HelloWorldController class.  The second part of the URL determines the action method on the class to execute.  So /HelloWorld/Index would  cause the Index method  of the HelloWorldController class  to execute. Notice that we only had to browse to /HelloWorld and  the Index method  was used by default. This is because a method named  Index is the default method  that will be called on a controller if one is not explicitly specified. The  third part of the URL segment ( Parameters) is for route data. We'll see route data later on in this  tutorial.

Browse to http://localhost:xxxx/HelloWorld/Welcome.  The Welcome method  runs and returns the string "This is the Welcome action method...". The  default MVC mapping is /[Controller]/[ActionName]/[Parameters].  For this URL, the controller is HelloWorld and Welcome is  the action method. You haven't used the [Parameters] part  of the URL yet.

Let's modify the example slightly so that you can pass some parameter information  from the URL to the controller (for example, /HelloWorld/Welcome?name=Scott&numtimes=4).  Change your Welcome method  to include two parameters as shown below. Note that the code uses the C# optional-parameter  feature to indicate that the  numTimes parameter should  default to 1 if no value is passed for that parameter.

public string Welcome(string name, int numTimes = 1) {
     return HttpUtility.HtmlEncode("Hello " + name + ", NumTimes is: " + numTimes);
}

Run your application and browse to the example URL (http://localhost:xxxx/HelloWorld/Welcome?name=Scott&numtimes=4).  You can try different values for name and numtimes in  the URL. The ASP.NET MVC model binding system automatically maps the named parameters from  the query string in the address bar to parameters in your method.

In the sample above, the URL segment ( Parameters) is not used, the name and numTimes parameters are passed as query strings.

Replace the Welcome method with the following code:

public string Welcome(string name, int ID = 1)
{
    return HttpUtility.HtmlEncode("Hello " + name + ", ID: " + ID);
}

Run the application and enter the following URL:  http://localhost:xxx/HelloWorld/Welcome/3?name=Rick

This time the third URL segment  matched the ID, because the  Welcome action method had a parameter  (ID) that matched the URL specification in the RegisterRoutes method.

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
    );
}

In ASP.NET MVC applications, it's more typical to pass in parameters as route  data (like we did with ID) than passing them as query strings. You could also  add a route to pass both the name and numtimes in  parameters as route data in the URL. In the App_Start\RouteConfig.cs  file, add the "Hello" route:

public class RouteConfig
{
   public static void RegisterRoutes(RouteCollection routes)
   {
      routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

      routes.MapRoute(
          name: "Default",
          url: "{controller}/{action}/{id}",
          defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
      );

      routes.MapRoute(
           name: "Hello",
           url: "{controller}/{action}/{name}/{id}"
       );
   }
}

Run the application and browse to /localhost:XXX/HelloWorld/Welcome/Scott/3.

For many MVC applications, the default route works fine. You'll learn later in this tutorial to pass data using the model binder, and you won't have to  modify the default route for that.

In these examples the controller has been doing the "VC" portion  of MVC — that is, the view and controller work. The controller is returning HTML  directly. Ordinarily you don't want controllers returning HTML directly, since  that becomes very cumbersome to code. Instead we'll typically use a separate  view template file to help generate the HTML response. Let's look next at how  we can do this.

By Rick Anderson, Rick Anderson works as a programmer writer for Microsoft, focusing on ASP.NET MVC, Windows Azure and Entity Framework. You can follow him on twitter via @RickAndMSFT.

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