The host or web host in ASP.NET is responsible for app startup and lifetime management. At a minimum, the host configures a server and a request processing pipeline. The host can also set up logging, dependency injection, and configuration

Set up a host

Create a host using an instance of IWebHostBuilder. This is typically performed in the app's entry point, the Main method.

                            public class Program
                            {
                                    public static void Main(string[] args)
                                    {
                                      CreateWebHostBuilder(args).Build().Run();
                                    }

                                    public static IWebHostBuilder CreateWebHostBuilder(string[] args) =>
                                    WebHost.CreateDefaultBuilder(args)
                                    .UseStartup();
                            }
                        

The code that calls CreateDefaultBuilder is in a method named CreateWebHostBuilder, which separates it from the code in Main that calls Run on the builder object.

CreateDefaultBuilder Roles

CreateDefaultBuilder has many roles in ASP.Net core:

The WebHostBuilder also allows us to load configuration from multiple sources into a final configuration representation of Key/Value pairs.

                           public class Program
                                {
                                    public static void Main(string[] args)
                                    {
                                       var host = new WebHostBuilder()
                                        .UseKestrel()
                                        .UseContentRoot(Directory.GetCurrentDirectory())
                                        .UseIISIntegration()
                                        .UseStartup()
                                        .Build();

                                        host.Run();
                                    }
                            }
                        

So why WebHostBuilder if CreateDefaultBuilder is there?

There is one main difference to be aware of. The HostBuilder doesn’t provide an extension method that allows you to use a startup class as we can with the WebHostBuilder. This decision was made primarily to avoid the need to create two separate DI containers behind the scenes. With the generic host, a single service collection is configured and then used to build a the final service provider.

The works extremely well for ASP.NET Core web applications, but there were no similar options in the framework for other types of application, until now!