Introduction to ASP.NET Core

ASP.NET Core is a significant redesign of ASP.NET. This topic introduces the new concepts in ASP.NET Core and explains how they help you develop modern web apps.

Sections:

  • What is ASP.NET Core?
  • Why build ASP.NET Core?
  • Application anatomy
  • Startup
  • Services
  • Middleware
  • Servers
  • Content root
  • Web root
  • Configuration
  • Environments
  • Build web UI and web APIs using ASP.NET Core MVC
  • Client-side development
  • Next steps

What is ASP.NET Core?

ASP.NET Core is a new open-source and cross-platform framework for building modern cloud based internet connected applications, such as web apps, IoT apps and mobile backends. ASP.NET Core apps can run on .NET Core or on the full .NET Framework. It was architected to provide an optimized development framework for apps that are deployed to the cloud or run on-premises. It consists of modular components with minimal overhead, so you retain flexibility while constructing your solutions. You can develop and run your ASP.NET Core apps cross-platform on Windows, Mac and Linux. ASP.NET Core is open source at GitHub.

Why build ASP.NET Core?

The first preview release of ASP.NET came out almost 15 years ago as part of the .NET Framework. Since then millions of developers have used it to build and run great web apps, and over the years we have added and evolved many capabilities to it.

ASP.NET Core has a number of architectural changes that result in a much leaner and modular framework. ASP.NET Core is no longer based on System.Web.dll. It is based on a set of granular and well factored NuGet packages. This allows you to optimize your app to include just the NuGet packages you need. The benefits of a smaller app surface area include tighter security, reduced servicing, improved performance, and decreased costs in a pay-for-what-you-use model.

With ASP.NET Core you gain the following foundational improvements:

  • A unified story for building web UI and web APIs
  • Integration of modern client-side frameworks and development workflows
  • A cloud-ready environment-based configuration system
  • Built-in dependency injection
  • New light-weight and modular HTTP request pipeline
  • Ability to host on IIS or self-host in your own process
  • Built on .NET Core, which supports true side-by-side app versioning
  • Ships entirely as NuGet packages
  • New tooling that simplifies modern web development
  • Build and run cross-platform ASP.NET apps on Windows, Mac and Linux
  • Open source and community focused

Application anatomy

An ASP.NET Core app is simply a console app that creates a web server in its Main method:

using System;
using Microsoft.AspNetCore.Hosting;

namespace aspnetcoreapp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            var host = new WebHostBuilder()
                .UseKestrel()
                .UseStartup<Startup>()
                .Build();

            host.Run();
        }
    }
}

Main uses WebHostBuilder, which follows the builder pattern, to create a web application host. The builder has methods that define the web server (for example UseKestrel) and the startup class (UseStartup). In the example above, the Kestrel web server is used, but other web servers can be specified. We’ll show more about UseStartup in the next section. WebHostBuilder provides many optional methods including UseIISIntegration for hosting in IIS and IIS Express and UseContentRootfor specifying the root content directory. The Build and Run methods build the IWebHost that will host the app and start it listening for incoming HTTP requests.

Startup

The UseStartup method on WebHostBuilder specifies the Startup class for your app.

public class Program
{
    public static void Main(string[] args)
    {
        var host = new WebHostBuilder()
            .UseKestrel()
            .UseStartup<Startup>()
            .Build();

        host.Run();
    }
}

The Startup class is where you define the request handling pipeline and where any services needed by the app are configured. The Startup class must be public and contain the following methods:

public class Startup
{
    public void ConfigureServices(IServiceCollection services)
    {
    }

    public void Configure(IApplicationBuilder app)
    {
    }
}
  • ConfigureServices defines the services (see Services below) used by your app (such as the ASP.NET MVC Core framework, Entity Framework Core, Identity, etc.)
  • Configure defines the middleware in the request pipeline
  • See Application Startup for more details

Services

A service is a component that is intended for common consumption in an application. Services are made available through dependency injection. ASP.NET Core includes a simple built-in inversion of control (IoC) container that supports constructor injection by default, but can be easily replaced with your IoC container of choice. In addition to its loose coupling benefit, DI makes services available throughout your app. For example, Logging is available throughout your app. SeeDependency Injection for more details.

Middleware

In ASP.NET Core you compose your request pipeline using Middleware. ASP.NET Core middleware performs asynchronous logic on an HttpContext and then either invokes the next middleware in the sequence or terminates the request directly. You generally “Use” middleware by invoking a corresponding UseXYZ extension method on the IApplicationBuilder in the Configure method.

ASP.NET Core comes with a rich set of prebuilt middleware:

  • Static files
  • Routing
  • Authentication

You can also author your own custom middleware.

You can use any OWIN-based middleware with ASP.NET Core. See Open Web Interface for .NET (OWIN) for details.

Servers

The ASP.NET Core hosting model does not directly listen for requests; rather it relies on an HTTPserver implementation to forward the request to the application. The forwarded request is wrapped as a set of feature interfaces that the application then composes into an HttpContext. ASP.NET Core includes a managed cross-platform web server, called Kestrel, that you would typically run behind a production web server like IIS or nginx.

Content root

The content root is the base path to any content used by the app, such as its views and web content. By default the content root is the same as application base path for the executable hosting the app; an alternative location can be specified with WebHostBuilder.

Web root

The web root of your app is the directory in your project for public, static resources like css, js, and image files. The static files middleware will only serve files from the web root directory (and sub-directories) by default. The web root path defaults to <content root>/wwwroot, but you can specify a different location using the WebHostBuilder.

Configuration

ASP.NET Core uses a new configuration model for handling simple name-value pairs. The new configuration model is not based on System.Configuration or web.config; rather, it pulls from an ordered set of configuration providers. The built-in configuration providers support a variety of file formats (XML, JSON, INI) and environment variables to enable environment-based configuration. You can also write your own custom configuration providers.

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