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Part 2, add a controller to an ASP.NET Core MVC app rick-anderson Part 2 of tutorial series on ASP.NET Core MVC. riande 01/23/2021
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tutorials/first-mvc-app/adding-controller contperf-fy21q3

Part 2, add a controller to an ASP.NET Core MVC app

By Rick Anderson

::: moniker range=">= aspnetcore-3.0"

The Model-View-Controller (MVC) architectural pattern separates an app into three main components: Model, View, and Controller. The MVC pattern helps you create apps that are more testable and easier to update than traditional monolithic apps.

MVC-based apps contain:

  • Models: 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 database.
  • Views: Views are the components that display the app's user interface (UI). Generally, this UI displays the model data.
  • Controllers: Classes that:
    • Handle browser requests.
    • Retrieve model data.
    • 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 URL segments and query-string values, and passes these values to the model. The model might use these values to query the database. For example:

  • https://localhost:5001/Home/Privacy: specifies the Home controller and the Privacy action.
  • https://localhost:5001/Movies/Edit/5: is a request to edit the movie with ID=5 using the Movies controller and the Edit action, which are detailed later in the tutorial.

Route data is explained later in the tutorial.

The MVC architectural pattern separates an app into three main groups of components: Models, Views, and Controllers. This pattern helps to achieve separation of concerns: The UI logic belongs in the view. Input logic belongs in the controller. Business logic belongs in the model. This separation helps manage complexity when building an app, because it enables 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.

These concepts are introduced and demonstrated in this tutorial series while building a movie app. The MVC project contains folders for the Controllers and Views.

Add a controller

Visual Studio

In the Solution Explorer, right-click Controllers > Add > Controller.

Solution Explorer, right click Controllers > Add > Controller

In the Add Scaffold dialog box, select MVC Controller - Empty.

Add MVC controller and name it

In the Add New Item - MvcMovie dialog, enter HelloWorldController.cs and select Add.

Visual Studio Code

Select the EXPLORER icon and then control-click (right-click) Controllers > New File and name the new file HelloWorldController.cs.

Contextual menu

Visual Studio for Mac

In Solution Explorer, right-click Controllers > Add > New File.

Contextual menu

Select ASP.NET Core and Controller Class.

Name the controller HelloWorldController.

Add MVC controller and name it

Replace the contents of Controllers/HelloWorldController.cs with the following:

[!code-csharp]

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.

An HTTP endpoint:

  • Is a targetable URL in the web application, such as https://localhost:5001/HelloWorld.
  • Combines:
    • The protocol used: HTTPS.
    • The network location of the web server, including the TCP port: localhost:5001.
    • The target URI: HelloWorld.

The first comment states this is an HTTP GET method that's invoked by appending /HelloWorld/ to the base URL.

The second comment specifies an HTTP GET method that's invoked by appending /HelloWorld/Welcome/ to the URL. Later on in the tutorial, the scaffolding engine is used to generate HTTP POST methods, which update data.

Run the app without the debugger.

Append "HelloWorld" to the path in the address bar. The Index method returns a string.

Browser window showing an app response of This is my default action

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

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

The routing format is set in the Configure method in Startup.cs file.

[!code-csharp]

When you browse to 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. In the preceding URL segments:

  • The first URL segment determines the controller class to run. So localhost:5001/HelloWorld maps to the HelloWorldController class.
  • The second part of the URL segment determines the action method on the class. So localhost:5001/HelloWorld/Index causes the Index method of the HelloWorldController class to run. Notice that you only had to browse to localhost:5001/HelloWorld and the Index method was called by default. Index is the default method that will be called on a controller if a method name isn't explicitly specified.
  • The third part of the URL segment ( id) is for route data. Route data is explained later in the tutorial.

Browse to: https://localhost:{PORT}/HelloWorld/Welcome. Replace {PORT} with your port number.

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. You haven't used the [Parameters] part of the URL yet.

Browser window showing an application response of This is the Welcome action method

Modify the code to pass some parameter information from the URL to the controller. For example, /HelloWorld/Welcome?name=Rick&numtimes=4.

Change the Welcome method to include two parameters as shown in the following code.

[!code-csharp]

The preceding code:

  • Uses the C# optional-parameter feature to indicate that the numTimes parameter defaults to 1 if no value is passed for that parameter.
  • Uses HtmlEncoder.Default.Encode to protect the app from malicious input, such as through JavaScript.
  • Uses Interpolated Strings in $"Hello {name}, NumTimes is: {numTimes}".

Run the app and browse to: https://localhost:{PORT}/HelloWorld/Welcome?name=Rick&numtimes=4. Replace {PORT} with your port number.

Try different values for name and numtimes in the URL. The MVC model binding system automatically maps the named parameters from the query string to parameters in the method. See Model Binding for more information.

Browser window showing an application response of Hello Rick, NumTimes is\: 4

In the previous image:

  • The URL segment Parameters isn't used.
  • The name and numTimes parameters are passed in the query string.
  • The ? (question mark) in the above URL is a separator, and the query string follows.
  • The & character separates field-value pairs.

Replace the Welcome method with the following code:

[!code-csharp]

Run the app and enter the following URL: https://localhost:{PORT}/HelloWorld/Welcome/3?name=Rick

In the preceding URL:

  • The third URL segment matched the route parameter id.
  • The Welcome method contains a parameter id that matched the URL template in the MapControllerRoute method.
  • The trailing ? starts the query string.

[!code-csharp]

In the preceding example:

  • The third URL segment matched the route parameter id.
  • The Welcome method contains a parameter id that matched the URL template in the MapControllerRoute method.
  • The trailing ? (in id?) indicates the id parameter is optional.

[!div class="step-by-step"] Previous: Get Started Next: Add a View

::: moniker-end

::: moniker range="< aspnetcore-3.0"

The Model-View-Controller (MVC) architectural pattern separates an app into three main components: Model, View, and Controller. The MVC pattern helps you create apps that are more testable and easier to update than traditional monolithic apps. MVC-based apps contain:

  • Models: 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 database.

  • Views: Views are the components that display the app's user interface (UI). Generally, this UI displays the model data.

  • Controllers: 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, https://localhost:5001/Home/About has route data of Home (the controller) and About (the action method to call on the home controller). https://localhost:5001/Movies/Edit/5 is a request to edit the movie with ID=5 using the movie controller. Route data is explained 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 cover these concepts in this tutorial series and show you how to use them to build a movie app. The MVC project contains folders for the Controllers and Views.

Add a controller

Visual Studio

  • In Solution Explorer, right-click Controllers > Add > Controller

    Contextual menu

  • In the Add Scaffold dialog box, select MVC Controller - Empty

    Add MVC controller and name it

  • In the Add Empty MVC Controller dialog, enter HelloWorldController and select ADD.

Visual Studio Code

Select the EXPLORER icon and then control-click (right-click) Controllers > New File and name the new file HelloWorldController.cs.

Contextual menu

Visual Studio for Mac

In Solution Explorer, right-click Controllers > Add > New File.

Contextual menu

Select ASP.NET Core and MVC Controller Class.

Name the controller HelloWorldController.

Add MVC controller and name it

Replace the contents of Controllers/HelloWorldController.cs with the following:

[!code-csharp]

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.

An HTTP endpoint is a targetable URL in the web application, such as https://localhost:5001/HelloWorld, and combines the protocol used: HTTPS, the network location of the web server (including the TCP port): localhost:5001 and the target URI HelloWorld.

The first comment states this is an HTTP GET method that's invoked by appending /HelloWorld/ to the base URL. The second comment specifies an HTTP GET method that's invoked by appending /HelloWorld/Welcome/ to the URL. Later on in the tutorial the scaffolding engine is used to generate HTTP POST methods which update data.

Run the app in non-debug mode and append "HelloWorld" to the path in the address bar. The Index method returns a string.

Browser window showing an application response of This is my default action

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

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

The routing format is set in the Configure method in Startup.cs file.

[!code-csharp]

When you browse to 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:{PORT}/HelloWorld maps to the HelloWorldController class. The second part of the URL segment determines the action method on the class. So localhost:{PORT}/HelloWorld/Index would cause the Index method of the HelloWorldController class to run. Notice that you only had to browse to localhost:{PORT}/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 isn't explicitly specified. The third part of the URL segment ( id) is for route data. Route data is explained later in the tutorial.

Browse to https://localhost:{PORT}/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. You haven't used the [Parameters] part of the URL yet.

Browser window showing an application response of This is the Welcome action method

Modify the code to pass some parameter information from the URL to the controller. For example, /HelloWorld/Welcome?name=Rick&numtimes=4. Change the Welcome method to include two parameters as shown in the following code.

[!code-csharp]

The preceding code:

  • Uses the C# optional-parameter feature to indicate that the numTimes parameter defaults to 1 if no value is passed for that parameter.
  • Uses HtmlEncoder.Default.Encode to protect the app from malicious input (namely JavaScript).
  • Uses Interpolated Strings in $"Hello {name}, NumTimes is: {numTimes}".

Run the app and browse to:

https://localhost:{PORT}/HelloWorld/Welcome?name=Rick&numtimes=4

(Replace {PORT} with your port number.) You can try different values for name and numtimes in the URL. The MVC model binding system automatically maps the named parameters from the query string in the address bar to parameters in your method. See Model Binding for more information.

Browser window showing an application response of Hello Rick, NumTimes is\: 4

In the image above, the URL segment (Parameters) isn't used, the name and numTimes parameters are passed in the query string. The ? (question mark) in the above URL is a separator, and the query string follows. The & character separates field-value pairs.

Replace the Welcome method with the following code:

[!code-csharp]

Run the app and enter the following URL: https://localhost:{PORT}/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.

[!code-csharp]

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 you typically use a separate Razor view template file to help generate the HTML response. You do that in the next tutorial.

[!div class="step-by-step"] Previous Next

::: moniker-end