asp.net core 系列 2 启动类 Startup.CS

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在探讨Startup启动类之前，我们先来了解下Asp.NET CORE 配置应用程序的执行顺序，如下图所示

与早期版本的 ASP.NET 对比，最显著的变化之一就是配置应用程序的方式， Global.asax、FilterConfig.cs 和 RouteConfig.cs 统统消失了，取而代之的是 Program.cs 和 Startup.cs。Program.cs 作为 Web 应用程序的默认入口，不做任何修改的情况下，会调用同目录下 Startup.cs 中的 ConfigureServices 方法和 Configure 方法。

首先我们来看下Progarm.cs，如下

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

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

View Code

我们看到Program中Main函数很像我们之前的Console应用程序，那么它当中的CreateWebHostBuilder究竟做了什么事情呢？既然.Net Core是开源的，我们看源码就方便了很多，接下来就从源码来了解下它究竟做了什么工作。

/// <summary>
        /// Initializes a new instance of the <see cref="WebHostBuilder"/> class with pre-configured defaults.
        /// </summary>
        /// <remarks>
        ///   The following defaults are applied to the returned <see cref="WebHostBuilder"/>:
        ///     use Kestrel as the web server and configure it using the application's configuration providers,
        ///     set the <see cref="IHostingEnvironment.ContentRootPath"/> to the result of <see cref="Directory.GetCurrentDirectory()"/>,
        ///     load <see cref="IConfiguration"/> from 'appsettings.json' and 'appsettings.[<see cref="IHostingEnvironment.EnvironmentName"/>].json',
        ///     load <see cref="IConfiguration"/> from User Secrets when <see cref="IHostingEnvironment.EnvironmentName"/> is 'Development' using the entry assembly,
        ///     load <see cref="IConfiguration"/> from environment variables,
        ///     load <see cref="IConfiguration"/> from supplied command line args,
        ///     configure the <see cref="ILoggerFactory"/> to log to the console and debug output,
        ///     and enable IIS integration.
        /// </remarks>
        /// <param name="args">The command line args.</param>
        /// <returns>The initialized <see cref="IWebHostBuilder"/>.</returns>
        public static IWebHostBuilder CreateDefaultBuilder(string[] args)
        {
            var builder = new WebHostBuilder();

            if (string.IsNullOrEmpty(builder.GetSetting(WebHostDefaults.ContentRootKey)))
            {
                builder.UseContentRoot(Directory.GetCurrentDirectory());
            }
            if (args != null)
            {
                builder.UseConfiguration(new ConfigurationBuilder().AddCommandLine(args).Build());
            }

            builder.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();
                logging.AddEventSourceLogger();
            }).
            UseDefaultServiceProvider((context, options) =>
            {
                options.ValidateScopes = context.HostingEnvironment.IsDevelopment();
            });

            ConfigureWebDefaults(builder);

            return builder;
        }

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internal static void ConfigureWebDefaults(IWebHostBuilder builder)
        {
            builder.UseKestrel((builderContext, options) =>
            {
                options.Configure(builderContext.Configuration.GetSection("Kestrel"));
            })
            .ConfigureServices((hostingContext, services) =>
            {
                // Fallback
                services.PostConfigure<HostFilteringOptions>(options =>
                {
                    if (options.AllowedHosts == null || options.AllowedHosts.Count == 0)
                    {
                        // "AllowedHosts": "localhost;127.0.0.1;[::1]"
                        var hosts = hostingContext.Configuration["AllowedHosts"]?.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries);
                        // Fall back to "*" to disable.
                        options.AllowedHosts = (hosts?.Length > 0 ? hosts : new[] { "*" });
                    }
                });
                // Change notification
                services.AddSingleton<IOptionsChangeTokenSource<HostFilteringOptions>>(
                            new ConfigurationChangeTokenSource<HostFilteringOptions>(hostingContext.Configuration));

                services.AddTransient<IStartupFilter, HostFilteringStartupFilter>();

                services.AddRouting();
            })
            .UseIIS()
            .UseIISIntegration();
        }

View Code

从源码中我们可以看到，CreateDefaultBuilder执行的任务有：

　　1、加载主机和应用程序的配置表信息

　　2、配置日志记录

　　3、设置Web服务器

　　4、设置Asp.Net Core应用程序的托管形式。

Asp.Net Core应用程序的托管形式，它有两种托管形式：进程内托管InProcess和进程外托管OutOfProcess。我们知道Asp.Net Core是可以自托管的，它默认托管形式就是InProcess。那么这两种方式的区别是什么呢?

InProcess：配置进程内托管在项目.csproj文件中 <AspNetCoreHostingModel>InProcess</AspNetCoreHostingModel>，在InProcess托管情况下，CreateDefaultBuilder()方法调用UseIIS()方法并在IIS工作进程（w3wp.exe或iisexpress.exe）内托管应用程序，从性能角度，InProcess托管比OutOfProcess托管提供了更高的请求吞吐量。

OutOfProcess：有2个Web服务器-内部Web服务器和外部Web服务器，内部Web服务器是Kestrel，托管进程是dotnet.exe；外部web服务器可以是iis，nginx，apache。

总之，我们可以初步理解为Program.cs 主要用来构造，配置，并启用项目所依赖的服务器，最后设置Asp.Net Core应用程序的托管形式。

在Program.cs中，我们发现它会引用Startup.CS，如下：

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

下面我们来看看启动类Startup.CS，如下：

    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.Configure<CookiePolicyOptions>(options =>
            {
                // This lambda determines whether user consent for non-essential cookies is needed for a given request.
                options.CheckConsentNeeded = context => true;
                options.MinimumSameSitePolicy = SameSiteMode.None;
            });


            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())
            {
                //如果是开发环境，允许抛出异常
                app.UseDeveloperExceptionPage();
            }
            else
            {
                //生产环境，直接错误页
                app.UseExceptionHandler("/Error");
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();
            app.UseCookiePolicy();

            app.UseMvc();
        }
    }