.NET WebAPI 启动顺序

TL;DR

本文为面试遇到启动顺序问题，先上结论，感兴趣可以往下看（会简单聊下源码，对比下 .NET Core 各个版本的区别）

1. ConfigureWebHostDefaults
2. ConfigureHostConfiguration
3. ConfigureAppConfiguration
4. • 以下根据语句的先后顺序执行
     • Startup.ConfigureServices
     • ConfigureLogging
     • ConfigureServices`
5. Startup.Configure

结果是很容易得出，dotnet new webapi, 然后对应位置console下就好了，但是为什么如此呢？
如果根据结果去推原因，并不是一个好的方法，你会用各种不知对错的想法去对应结果（不只是代码，人生也是如果 就如各种畅销书/垃圾水文，根据人的成功，去证明人的各种小习惯的重要性）。所以本文试图通过两个方式探究下原因。

Program, Startup 变化

.NET Core 1.0

Program.cs

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

    host.Run();
}

Startup.cs

public Startup(IHostingEnvironment env)
{
    var builder = new ConfigurationBuilder()
        .SetBasePath(env.ContentRootPath)
        .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
        .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
        .AddEnvironmentVariables();
    Configuration = builder.Build();
}

public IConfigurationRoot Configuration { get; }

// This method gets called by the runtime. Use this method to add services to the container.
public void ConfigureServices(IServiceCollection services)
{
    // Add framework services.
    services.AddMvc();
}

// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
{
    loggerFactory.AddConsole(Configuration.GetSection("Logging"));
    loggerFactory.AddDebug();

    if (env.IsDevelopment())
    {
        app.UseDeveloperExceptionPage();
    }

    app.UseMvc();
}

.NET Core 1.1

Program.cs

与 1.0 一致

Startup.cs

与 1.0 一致

.NET Core 2.0

Program.cs

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

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

嗯，2.0的时候已经清爽了不少，把那些默认的中间件的使用都放在了 CreateDefaultBuilder 中，我们来看下这个方法做了哪些事情(代码在下边)，可以看到UseKestrel，UseIIS都是在这个方法中实现的。并且把 .NET Core 1 中Startup做的config部分工作放在了这里，还有log部分，也是在此实现。

public static IWebHostBuilder CreateDefaultBuilder(string[] args)
{
    return new WebHostBuilder().UseKestrel().UseContentRoot(Directory.GetCurrentDirectory()).ConfigureAppConfiguration(delegate (WebHostBuilderContext hostingContext, IConfigurationBuilder config)
    {
        IHostingEnvironment hostingEnvironment = hostingContext.HostingEnvironment;
        config.AddJsonFile("appsettings.json", optional: true, reloadOnChange: true).AddJsonFile($"appsettings.{hostingEnvironment.EnvironmentName}.json", optional: true, reloadOnChange: true);
        if (hostingEnvironment.IsDevelopment())
        {
            Assembly assembly = Assembly.Load(new AssemblyName(hostingEnvironment.ApplicationName));
            if (assembly != null)
            {
                config.AddUserSecrets(assembly, optional: true);
            }
        }

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

Startup.cs

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseMvc();
    }
}

因为在main函数中就把config配置好了，所以Startup的ctor参数也从 IHostingEnvironment -> IConfiguration， 从这之后基本就没有太大的变动了

BTW 常看报文的肯定经常看到这个 Server: Kestrel ， 有心的人估计也查过这个，知道这个是与 Nginx,IIS，Apache一样的，用来负载你的web程序，但是有一大部分人都说自己在程序中并没用用过这个，部署的时候也一直是 nignx,IIS, 实际上是微软已经默认使用了。

.NET Core 2.1

Program.cs

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

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

换了下函数的返回值 IWebHost -> IWebHostBuilder

Startup.cs

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddMvc().SetCompatibilityVersion(CompatibilityVersion.Version_2_1);
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }
        else
        {
            app.UseHsts();
        }

        app.UseHttpsRedirection();
        app.UseMvc();
    }
}

SetCompatibilityVersion 看名字也知道是为了兼容性，不用管
多了一个HSTS 的中间件（为了强制使用HTTPS），估计是 RFC 有啥新出条例，所以项目默认支持
UseHttpsRedirection 一样是为了安全

这一版基本没什么变化

.NET Core 2.2

Program.cs

无变化

Startup.cs

基本无变化

.NET Core 3.1

Program.cs

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

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup<Startup>();
            });
}

IWebHostBuilder -> IHostBuilder 这样可以更好的将 ASP.NET Core 应用与非 Web 特定的其他服务器方案集成 ,通过 Hosting Exception， create default build, config web host default 启用 API 。

这一部分更多是，微软对于 .NET Core 代码类库的变化，整体的封装都做了很大的调整（v2 的时候出了Microsoft.AspNetCore.All,基本引入的所有的必需包，当然会导致用不到的包也引入了，项目加载必然会慢一点，V3的时候做了调整 ）

Startup.cs

public class Startup
{
    public Startup(IConfiguration configuration)
    {
        Configuration = configuration;
    }

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers();
    }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
    {
        if (env.IsDevelopment())
        {
            app.UseDeveloperExceptionPage();
        }

        app.UseHttpsRedirection();

        app.UseRouting();

        app.UseAuthorization();

        app.UseEndpoints(endpoints =>
        {
            endpoints.MapControllers();
        });
    }
}

这次是将 AddMvc -> AddControllers, 即默认的注入变为Controll，不再包括视图（view，page），也是因为这个变化，可以看到下边多了一些中间件的注入（route,authorization,endpoint）。

为什么这个时候多出了这些呢？ 如果看下之前代码，会发现的UseMvc()中route,authorization,endpoint这些东西实际上在MVC中都有实现。

可以说这次的更新，微软更多的是把默认配置变得更轻巧，灵活一点。（当然之前的useMvc也是可以正常用的，注意 微软只是改的默认写法，而不是规定你必须这样做）

.NET Core 5

Program.cs

无变化

Startup.cs

无变化

PS. 从这一代，webapi项目默认集成了OpenAPI(Swagger)

.NET Core 6

Program.cs

无变化

Startup.cs

无变化

瞧瞧源码

先把相关的方法都加上

public class Program
{
    public static void Main(string[] args) => CreateHostBuilder(args).Build().Run();

    public static IHostBuilder CreateHostBuilder(string[] args) => Host.CreateDefaultBuilder(args)
        .ConfigureAppConfiguration(builder =>
        {
            Console.WriteLine("ConfigureAppConfiguration");
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            Console.WriteLine(" ConfigureWebHostDefaults");
            webBuilder.UseStartup<Startup>();
        })
        .ConfigureLogging(log =>
        {
            Console.WriteLine("ConfigureLogging");
        })
        .ConfigureServices(service =>
        {
            Console.WriteLine("ConfigureServices");
        })
        .ConfigureHostConfiguration(config =>
        {
            Console.WriteLine("ConfigureHostConfiguration");
        });
}

ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
Startup
Startup.ConfigureServices
ConfigureLogging
ConfigureServices
service:3226198
service2:29035785
Startup.Configure