ASP.NET Core

The ASP.NET Core MVC framework is a lightweight, open source, highly testable presentation framework optimized for use with ASP.NET Core.

ASP.NET Core MVC provides a patterns-based way to build dynamic websites that enables a clean separation of concerns. It gives you full control over markup, supports TDD-friendly development and uses the latest web standards.

Features

ASP.NET Core MVC includes the following:

  • Routing
  • Model binding
  • Model validation
  • Dependency injection
  • Filters
  • Areas
  • Web APIs
  • Testability
  • Razor view engine
  • Strongly typed views
  • Tag Helpers
  • View Components

Routing

ASP.NET Core MVC is built on top of ASP.NET Core’s routing, a powerful URL-mapping component that lets you build applications that have comprehensible and searchable URLs. This enables you to define your application’s URL naming patterns that work well for search engine optimization (SEO) and for link generation, without regard for how the files on your web server are organized. You can define your routes using a convenient route template syntax that supports route value constraints, defaults and optional values.

Convention-based routing enables you to globally define the URL formats that your application accepts and how each of those formats maps to a specific action method on given controller. When an incoming request is received, the routing engine parses the URL and matches it to one of the defined URL formats, and then calls the associated controller’s action method.

routes.MapRoute(name: "Default", template: "{controller=Home}/{action=Index}/{id?}");

Attribute routing enables you to specify routing information by decorating your controllers and actions with attributes that define your application’s routes. This means that your route definitions are placed next to the controller and action with which they’re associated.

[Route("api/[controller]")]
public class ProductsController : Controller
{
  [HttpGet("{id}")]
  public IActionResult GetProduct(int id)
  {
    ...
  }
}

Model binding

ASP.NET Core MVC model binding converts client request data (form values, route data, query string parameters, HTTP headers) into objects that the controller can handle. As a result, your controller logic doesn’t have to do the work of figuring out the incoming request data; it simply has the data as parameters to its action methods.

public async Task<IActionResult> Login(LoginViewModel model, string returnUrl = null) { ... }

Model validation

ASP.NET Core MVC supports validation by decorating your model object with data annotation validation attributes. The validation attributes are checked on the client side before values are posted to the server, as well as on the server before the controller action is called.

using System.ComponentModel.DataAnnotations;
public class LoginViewModel
{
    [Required]
    [EmailAddress]
    public string Email { get; set; }

    [Required]
    [DataType(DataType.Password)]
    public string Password { get; set; }

    [Display(Name = "Remember me?")]
    public bool RememberMe { get; set; }
}

A controller action:

public async Task<IActionResult> Login(LoginViewModel model, string returnUrl = null)
{
    if (ModelState.IsValid)
    {
      // work with the model
    }
    // At this point, something failed, redisplay form
    return View(model);
}

The framework handles validating request data both on the client and on the server. Validation logic specified on model types is added to the rendered views as unobtrusive annotations and is enforced in the browser with jQuery Validation.

Dependency injection

ASP.NET Core has built-in support for dependency injection (DI). In ASP.NET Core MVC, controllers can request needed services through their constructors, allowing them to follow the Explicit Dependencies Principle.

Your app can also use dependency injection in view files, using the @inject directive:

@inject SomeService ServiceName
<!DOCTYPE html>
<html lang="en">
<head>
    <title>@ServiceName.GetTitle</title>
</head>
<body>
    <h1>@ServiceName.GetTitle</h1>
</body>
</html>

Filters

Filters help developers encapsulate cross-cutting concerns, like exception handling or authorization. Filters enable running custom pre- and post-processing logic for action methods, and can be configured to run at certain points within the execution pipeline for a given request. Filters can be applied to controllers or actions as attributes (or can be run globally). Several filters (such as Authorize) are included in the framework. [Authorize] is the attribute that is used to create MVC authorization filters.

[Authorize]
   public class AccountController : Controller
   {

Areas

Areas provide a way to partition a large ASP.NET Core MVC Web app into smaller functional groupings. An area is an MVC structure inside an application. In an MVC project, logical components like Model, Controller, and View are kept in different folders, and MVC uses naming conventions to create the relationship between these components. For a large app, it may be advantageous to partition the app into separate high level areas of functionality. For instance, an e-commerce app with multiple business units, such as checkout, billing, and search etc. Each of these units have their own logical component views, controllers, and models.

Web APIs

In addition to being a great platform for building web sites, ASP.NET Core MVC has great support for building Web APIs. You can build services that reach a broad range of clients including browsers and mobile devices.

The framework includes support for HTTP content-negotiation with built-in support to format data as JSON or XML. Write custom formatters to add support for your own formats.

Use link generation to enable support for hypermedia. Easily enable support for cross-origin resource sharing (CORS) so that your Web APIs can be shared across multiple Web applications.

Testability

The framework’s use of interfaces and dependency injection make it well-suited to unit testing, and the framework includes features (like a TestHost and InMemory provider for Entity Framework) that make integration tests quick and easy as well.

Razor view engine

ASP.NET Core MVC views use the Razor view engine to render views. Razor is a compact, expressive and fluid template markup language for defining views using embedded C# code. Razor is used to dynamically generate web content on the server. You can cleanly mix server code with client side content and code.text

<ul>
  @for (int i = 0; i < 5; i++) {
    <li>List item @i</li>
  }
</ul>

Using the Razor view engine you can define layouts, partial views and replaceable sections.

Strongly typed views

Razor views in MVC can be strongly typed based on your model. Controllers can pass a strongly typed model to views enabling your views to have type checking and IntelliSense support.

For example, the following view renders a model of type IEnumerable<Product>:

@model IEnumerable<Product>
<ul>
    @foreach (Product p in Model)
    {
        <li>@p.Name</li>
    }
</ul>

Tag Helpers

Tag Helpers enable server side code to participate in creating and rendering HTML elements in Razor files. You can use tag helpers to define custom tags (for example, <environment>) or to modify the behavior of existing tags (for example, <label>). Tag Helpers bind to specific elements based on the element name and its attributes. They provide the benefits of server-side rendering while still preserving an HTML editing experience.

There are many built-in Tag Helpers for common tasks – such as creating forms, links, loading assets and more – and even more available in public GitHub repositories and as NuGet packages. Tag Helpers are authored in C#, and they target HTML elements based on element name, attribute name, or parent tag. For example, the built-in LinkTagHelper can be used to create a link to the Login action of the AccountsController:

<p>
    Thank you for confirming your email.
    Please <a asp-controller="Account" asp-action="Login">Click here to Log in</a>.
</p>

The EnvironmentTagHelper can be used to include different scripts in your views (for example, raw or minified) based on the runtime environment, such as Development, Staging, or Production:

<environment names="Development">
    <script src="~/lib/jquery/dist/jquery.js"></script>
</environment>
<environment names="Staging,Production">
    <script src="https://ajax.aspnetcdn.com/ajax/jquery/jquery-2.1.4.min.js"
            asp-fallback-src="~/lib/jquery/dist/jquery.min.js"
            asp-fallback-test="window.jQuery">
    </script>
</environment>

Tag Helpers provide an HTML-friendly development experience and a rich IntelliSense environment for creating HTML and Razor markup. Most of the built-in Tag Helpers target existing HTML elements and provide server-side attributes for the element.

View Components

View Components allow you to package rendering logic and reuse it throughout the application. They’re similar to partial views, but with associated logic.

View components are similar to partial views, but they’re much more powerful. View components don’t use model binding, and only depend on the data provided when calling into it. This article was written using ASP.NET Core MVC, but view components also work with Razor Pages.

A view component:

  • Renders a chunk rather than a whole response.
  • Includes the same separation-of-concerns and testability benefits found between a controller and view.
  • Can have parameters and business logic.
  • Is typically invoked from a layout page.

View components are intended anywhere you have reusable rendering logic that’s too complex for a partial view, such as:

  • Dynamic navigation menus
  • Tag cloud (where it queries the database)
  • Login panel
  • Shopping cart
  • Recently published articles
  • Sidebar content on a typical blog
  • A login panel that would be rendered on every page and show either the links to log out or log in, depending on the log in state of the user

A view component consists of two parts: the class (typically derived from ViewComponent) and the result it returns (typically a view). Like controllers, a view component can be a POCO, but most developers will want to take advantage of the methods and properties available by deriving from ViewComponent.

Compatibility version

The SetCompatibilityVersion method allows an app to opt-in or opt-out of potentially breaking behavior changes introduced in ASP.NET Core MVC 2.1 or later.

The SetCompatibilityVersion method allows an app to opt-in or opt-out of potentially breaking behavior changes introduced in ASP.NET Core MVC 2.1 or later. These potentially breaking behavior changes are generally in how the MVC subsystem behaves and how your code is called by the runtime. By opting in, you get the latest behavior, and the long-term behavior of ASP.NET Core.

The following code sets the compatibility mode to ASP.NET Core 2.2:

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        .SetCompatibilityVersion(CompatibilityVersion.Version_2_2);
}

We recommend you test your app using the latest version (CompatibilityVersion.Version_2_2). We anticipate that most apps won’t have breaking behavior changes using the latest version.

Apps that call SetCompatibilityVersion(CompatibilityVersion.Version_2_0) are protected from potentially breaking behavior changes introduced in the ASP.NET Core 2.1 MVC and later 2.x versions. This protection:

  • Does not apply to all 2.1 and later changes, it’s targeted to potentially breaking ASP.NET Core runtime behavior changes in the MVC subsystem.
  • Does not extend to the next major version.

The default compatibility for ASP.NET Core 2.1 and later 2.x apps that do not call SetCompatibilityVersion is 2.0 compatibility. That is, not calling SetCompatibilityVersion is the same as calling SetCompatibilityVersion(CompatibilityVersion.Version_2_0).

The following code sets the compatibility mode to ASP.NET Core 2.2, except for the following behaviors:

  • AllowCombiningAuthorizeFilters
  • InputFormatterExceptionPolicy
public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc()
        // Include the 2.2 behaviors
        .SetCompatibilityVersion(CompatibilityVersion.Version_2_2)
        // Except for the following.
        .AddMvcOptions(options =>
        {
            // Don't combine authorize filters (keep 2.0 behavior).
            options.AllowCombiningAuthorizeFilters = false;
            // All exceptions thrown by an IInputFormatter are treated
            // as model state errors (keep 2.0 behavior).
            options.InputFormatterExceptionPolicy =
                InputFormatterExceptionPolicy.AllExceptions;
        });
}

For apps that encounter breaking behavior changes, using the appropriate compatibility switches:

  • Allows you to use the latest release and opt out of specific breaking behavior changes.
  • Gives you time to update your app so it works with the latest changes.

The MvcOptions class source comments have a good explanation of what changed and why the changes are an improvement for most users.

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