By [Rick Anderson](https://twitter.com/RickAndMSFT)
The Model-View-Controller (MVC) architectural pattern separates an app into three main components: **M**odel, **V**iew, and **C**ontroller. The MVC pattern helps you create apps that are more testable and easier to update than traditional monolithic apps. MVC-based apps contain:
* **M**odels: Classes that represent the data of the app. The model classes use validation logic to enforce business rules for that data. Typically, model objects retrieve and store model state in a database. In this tutorial, a `Movie` model retrieves movie data from a database, provides it to the view or updates it. Updated data is written to a SQL Server database.
* **V**iews: Views are the components that display the app's user interface (UI). Generally, this UI displays the model data.
* **C**ontrollers: Classes that handle browser requests. They retrieve model data and call view templates that return a response. In an MVC app, the view only displays information; the controller handles and responds to user input and interaction. For example, the controller handles route data and query-string values, and passes these values to the model. The model might use these values to query the database. For example, `http://localhost:1234/Home/About` has route data of `Home` (the controller) and `About` (the action method to call on the home controller). `http://localhost:1234/Movies/Edit/5` is a request to edit the movie with ID=5 using the movie controller. We'll talk about route data later in the tutorial.
The MVC pattern helps you create apps that separate the different aspects of the app (input logic, business logic, and UI logic), while providing a loose coupling between these elements. The pattern specifies where each kind of logic should be located in the app. The UI logic belongs in the view. Input logic belongs in the controller. Business logic belongs in the model. This separation helps you manage complexity when you build an app, because it enables you to work on one aspect of the implementation at a time without impacting the code of another. For example, you can work on the view code without depending on the business logic code.
We'll be covering all these concepts in this tutorial series and show you how to use them to build a simple movie app. The MVC project currently contains folders for the *Controllers* and *Views*. A *Models* folder will be added in a later step.
* In **VS Code**, select the **EXPLORER** icon and then control-click (right-click) **Controllers > New File**
Every `public` method in a controller is callable as an HTTP endpoint. In the sample above, both methods return a string. Note the comments preceding each method.
The first comment states this is an [HTTP GET](http://www.w3schools.com/tags/ref_httpmethods.asp) method that is invoked by appending "/HelloWorld/" to the base URL. The second comment specifies an [HTTP GET](http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html) method that is invoked by appending "/HelloWorld/Welcome/" to the URL. Later on in the tutorial we'll use the scaffolding engine to generate `HTTP POST` methods.
Run the app and navigate to http://localhost:5000/HelloWorld
The `Index` method returns a string. You told the system to return some HTML, and it did!
MVC invokes controller classes (and the action methods within them) depending on the incoming URL. The default [URL routing logic](../../mvc/controllers/routing.md) used by MVC uses a format like this to determine what code to invoke:
`/[Controller]/[ActionName]/[Parameters]`
You set the format for routing in the *Startup.cs* file.
When you run the app and don't supply any URL segments, it defaults to the "Home" controller and the "Index" method specified in the template line highlighted above.
The first URL segment determines the controller class to run. So `localhost:5000/HelloWorld` maps to the `HelloWorldController` class. The second part of the URL segment determines the action method on the class. So `localhost:5000/HelloWorld/Index` would cause the `Index` method of the `HelloWorldController` class to run. Notice that we only had to browse to `localhost:5000/HelloWorld` and the `Index` method was called by default. This is because `Index` is the default method that will be called on a controller if a method name is not explicitly specified. The third part of the URL segment ( `id`) is for route data. We'll see route data later on in this tutorial.
Browse to http://localhost:5000/HelloWorld/Welcome
The `Welcome` method runs and returns the string "This is the Welcome action method...". For this URL, the controller is `HelloWorld` and `Welcome` is the action method. We 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 the `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 defaults to 1 if no value is passed for that parameter.
The code above uses `HtmlEncoder.Default.Encode` to protect the app from malicious input (namely JavaScript). It also uses [Interpolated Strings](https://docs.microsoft.com/dotnet/articles/csharp/language-reference/keywords/interpolated-strings).
You can try different values for `name` and `numtimes` in the URL. The MVC [model binding](xref:mvc/models/model-binding) system automatically maps the named parameters from the query string in the address bar to parameters in your method. See [Model Binding](xref:mvc/models/model-binding) for more information.
In the sample above, the URL segment (`Parameters`) is not used, the `name` and `numTimes` parameters are passed as [query strings](http://en.wikipedia.org/wiki/Query_string). The `?` (question mark) in the above URL is a separator, and the query strings follow. The `&` character separates query strings.
Replace the `Welcome` method with the following code:
Run the app and enter the following URL: `http://localhost:xxx/HelloWorld/Welcome/3?name=Rick`
This time the third URL segment matched the route parameter `id`. The `Welcome` method contains a parameter `id` that matched the URL template in the `MapRoute` method. The trailing `?` (in `id?`) indicates the `id` parameter is optional.
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. Generally you don't want controllers returning HTML directly, since that becomes very cumbersome to code and maintain. Instead we'll typically use a separate Razor view template file to help generate the HTML response.
