ASP.NET Core fundamentals overview

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 UseContentRoot for specifying the root content directory. The Build and Run methods build the IWebHost object 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

For more information, see Application startup.

Services

A service is a component that is intended for common consumption in an application. Services are made available through dependency injection (DI). ASP.NET Core includes a simple built-in inversion of control (IoC) container that supports constructor injection by default. The built-in container can be easily replaced with your 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.

For more information, see Dependency injection .

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 taking a dependency on a NuGet package and invoking a corresponding UseXYZ extension method on the IApplicationBuilder in the Configure method.

ASP.NET Core comes with a rich set of built-in middleware:

You can use any OWIN-based middleware with ASP.NET Core, and you can write your own custom middleware.

For more information, see Middleware and Open Web Interface for .NET (OWIN).

Servers

The ASP.NET Core hosting model does not directly listen for requests; rather it relies on an HTTP server 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.

For more information, see Servers and Hosting.

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 /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.

For more information, see Configuration.

Environments

Environments, like "Development" and "Production", are a first-class notion in ASP.NET Core and can be set using environment variables.

For more information, see Working with Multiple Environments.

.NET Core vs. .NET Framework runtime

An ASP.NET Core app can use the .NET Core or .NET Framework runtime. For more information, see Choosing between .NET Core and .NET Framework.