2021-04-17发表2021-11-01更新10 分钟读完 (大约1538个字)

.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）

ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
- 以下根据语句的先后顺序执行
  - Startup.ConfigureServices
  - ConfigureLogging
  - ConfigureServices`
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

.NET WebAPI 启动顺序

Program, Startup 变化

.NET Core 1.0

Program.cs

Startup.cs

.NET Core 1.1

Program.cs

Startup.cs

.NET Core 2.0

Program.cs

Startup.cs

.NET Core 2.1

Program.cs

Startup.cs

.NET Core 2.2

Program.cs

Startup.cs

.NET Core 3.1

Program.cs

Startup.cs

.NET Core 5

Program.cs

Startup.cs

.NET Core 6

Program.cs

Startup.cs

瞧瞧源码

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