ASP.NET Core - Program.cs

ASP.NET Core web application is actually a console project which starts executing from the entry point public static void Main() in Program class where we can create a host for the web application.

Setup Host in ASP.NET Core 2.x

The following is the Program class in ASP.NET Core 2.x:

Program.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting;

namespace MyFirstCoreApp
{
    public class Program
    {
        public static void Main(string[] args)
        {
            BuildWebHost(args).Run();
        }

        public static IWebHost BuildWebHost(string[] args) =>
            WebHost.CreateDefaultBuilder(args)
                .UseStartup<Startup>()
                .Build();
    }
}

As you can see above, the Main() method calls method expression BuildWebHost() to build web host with pre-configured defaults. The BuildWebHost expression can also be written as a method that returns IWebHost as shown below.

public static void Main(string[] args)
{
    BuildWebHost(args).Run();
}

public static IWebHost BuildWebHost(string[] args) 
{
    return WebHost.CreateDefaultBuilder(args)
        .UseStartup<Startup>()
        .Build();
}

Let's understand hosting steps.

The WebHost is a static class which can be used for creating an instance of IWebHost and IWebHostBuilder with pre-configured defaults. The CreateDefaultBuilder() method creates a new instance of WebHostBuilder with pre-configured defaults. Internally, it configures Kestrel, IISIntegration and other configurations. The following is CreateDefaultBuilder() method.

CreateDefaultBuilder()

public static IWebHostBuilder CreateDefaultBuilder(string[] args)
{
    var builder = new WebHostBuilder()
        .UseKestrel()
        .UseContentRoot(Directory.GetCurrentDirectory())
        .ConfigureAppConfiguration((hostingContext, config) =>
        {
            var env = hostingContext.HostingEnvironment;

            config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
                    .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true, reloadOnChange: true);

            if (env.IsDevelopment())
            {
                var appAssembly = Assembly.Load(new AssemblyName(env.ApplicationName));
                if (appAssembly != null)
                {
                    config.AddUserSecrets(appAssembly, optional: true);
                }
            }

            config.AddEnvironmentVariables();

            if (args != null)
            {
                config.AddCommandLine(args);
            }
        })
        .ConfigureLogging((hostingContext, logging) =>
        {
            logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
            logging.AddConsole();
            logging.AddDebug();
        })
        .UseIISIntegration()
        .UseDefaultServiceProvider((context, options) =>
        {
            options.ValidateScopes = context.HostingEnvironment.IsDevelopment();
        });

    return builder;
}

As you can see above, the CreateDefaultBuilder method creates an instance of WebHostBuilder and sets up Kestrel, content root directory, IIS integration.

Kestrel is an open-source, cross-platform web server for ASP.NET Core. It is designed to be used behind proxy because it has not yet matured to be exposed as a full-fledge web server.

It also calls ConfigureAppConfiguration() to load configurations from appsettings.json files, environment variables and user secrets. The ConfigureLogging() method setup logging to console and debug window.