AspNetCore的Program类

• IHostBuilder中的相关配置方法

1. ConfigureWebHostDefaults：该方法注册了应用程序必要的几个组件，比如配置的组件、容器的组件等。
2. ConfigureHostConfiguration：该方法配置了应用程序启动时需要的配置，比如端口和url
3. ConfigureAppConfiguration：该方法可以让我们来嵌入我们自己的配置文件，供应用程序读取
4. ConfigureServices：该方法是用来往容器里注入我们应用需要的组件

/*
执行的顺序是：
ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
Startup.ConfigureServices
ConfigureServices
Startup.Configure
*/
public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureAppConfiguration(builder =>
                {
                    Console.WriteLine("ConfigureAppConfiguration");
                })
                .ConfigureServices(service =>
                {
                    Console.WriteLine("ConfigureServices");
                })
                .ConfigureHostConfiguration(builder =>
                {
                    Console.WriteLine("ConfigureHostConfiguration");
                })
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    Console.WriteLine("ConfigureWebHostDefaults");
                    webBuilder.UseStartup<Startup>();
                });

• 通过CreateHostBuilder(args).Build().Run()，就可以在IHostBuilder配置完后运行整个项目

/*
执行的顺序是：
ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
webBuilder.ConfigureServices
ConfigureServices
webBuilder.Configure
*/
public static IHostBuilder CreateHostBuilder(string[] args) =>
            Host.CreateDefaultBuilder(args)
                .ConfigureWebHostDefaults(webBuilder =>
                {
                    Console.WriteLine("ConfigureWebHostDefaults");
                    //webBuilder.UseStartup<Startup>();
                    webBuilder.ConfigureServices(services =>
                    {
                        Console.WriteLine("webBuilder.ConfigureServices");
                    });
                    webBuilder.Configure(app =>
                    {
                        Console.WriteLine("webBuilder.Configure");

                        app.UseRouting();

                        app.UseEndpoints(endpoints =>
                        {
                            endpoints.MapGet("/", async context =>
                            {
                                await context.Response.WriteAsync("Hello World!");
                            });
                        });
                    });
                })
                .ConfigureServices(service =>
                {
                    Console.WriteLine("ConfigureServices");
                })
                .ConfigureHostConfiguration(builder =>
                {
                    Console.WriteLine("ConfigureHostConfiguration");
                }).ConfigureAppConfiguration(builder =>
                {
                    Console.WriteLine("ConfigureAppConfiguration");
                });
    }

• 从这段代码不难看出，Startup类并不是必需的，它可以直接写在webBuilder下，但通常使用Startup类可以使我们的代码更有结构。