前言

本来想整理到<<重新整理.net core 计1400篇>>里面去，但是后来一想，整理 .net core 实践篇 是偏于实践，故而分开。

因为是重新整理，那么就从配置开始整理。以下只是个人理解，如有错误，望请指点谢谢。

正文

在我们创建好一个应用的时候，那么出现在我们视野的是一个这样的东西:

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>();
			});
}

看到这种:

CreateHostBuilder(args).Build().Run();

显然是建设者模式。

那么前面的基本就是在做一个构建。

既然是一个建设者模式，那么就来看一下这个的构建器是什么?

是一个叫做IHOSTBUILDER 的东西哈，而显然从命名上看这是一个接口哈。那么看下这个接口是啥:

跟配置相关得到也就是那几个Configure开头的那4个东西。

后面两个UseServiceProviderFactory后面系列再说。

//
// 摘要:
//     Sets up the configuration for the remainder of the build process and application.
//     This can be called multiple times and the results will be additive. The results
//     will be available at Microsoft.Extensions.Hosting.HostBuilderContext.Configuration
//     for subsequent operations, as well as in Microsoft.Extensions.Hosting.IHost.Services.
//
// 参数:
//   configureDelegate:
//     The delegate for configuring the Microsoft.Extensions.Configuration.IConfigurationBuilder
//     that will be used to construct the Microsoft.Extensions.Configuration.IConfiguration
//     for the application.
//
// 返回结果:
//     The same instance of the Microsoft.Extensions.Hosting.IHostBuilder for chaining.
IHostBuilder ConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate);

这个函数的大致意思是说构件一个IConfigurationBuilder，这个the remainder of the build process and application.的配置。

// 摘要:
//     Enables configuring the instantiated dependency container. This can be called
//     multiple times and the results will be additive.
//
// 参数:
//   configureDelegate:
//     The delegate which configures the builder.
//
// 类型参数:
//   TContainerBuilder:
//     The type of builder.
//
// 返回结果:
//     The same instance of the Microsoft.Extensions.Hosting.IHostBuilder for chaining.
IHostBuilder ConfigureContainer<TContainerBuilder>(Action<HostBuilderContext, TContainerBuilder> configureDelegate);

依赖容器相关的哈。

ConfigureHostConfiguration 和 ConfigureServices 这两个就不贴了，分别是一些坏境配置和服务配置。

那么启动一下，看下他们的执行顺序是否和我们代码的书写顺序是否相同，也就是说不管我如何调换顺序他们的执行都是按照某种规则。

那么这里加上log，通过log的方式来查看执行顺序，如下:

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");
			}).ConfigureServices(builder =>
			{
				Console.WriteLine("ConfigureServices");
			}).ConfigureHostConfiguration(builder =>
			{
				Console.WriteLine("ConfigureHostConfiguration");
			})
			.ConfigureWebHostDefaults(webBuilder =>
			{
				Console.WriteLine("ConfigureWebHostDefaults");
				webBuilder.UseStartup<Startup>();
			});
}

startup.cs

public class Startup
{
	public IConfiguration Configuration { get; }

	public Startup(IConfiguration configuration)
	{
		Console.WriteLine("Startup");
		Configuration = configuration;
	}

	// This method gets called by the runtime. Use this method to add services to the container.
	// For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
	public void ConfigureServices(IServiceCollection services)
	{
		Console.WriteLine("Startup.ConfigureServices");
		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)
	{
		Console.WriteLine("Startup.Configure");
		if (env.IsDevelopment())
		{
			app.UseDeveloperExceptionPage();
		}

		app.UseRouting();

		app.UseEndpoints(endpoints =>
		{
			endpoints.MapGet("/", async context =>
			{
				await context.Response.WriteAsync("Hello World!");
			});
		});
	}
}

这里可以发现执行顺序和我们的代码的书写顺序并不是一致的。

那么我们再次做一个小小的调换，那么会怎么样呢？

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

发现变化的只有Startup.ConfigureServices 和 ConfigureServices。

这个时候我们大体猜出来了这个启动顺序是按照某种执行顺序执行，且Startup.ConfigureServices 和 ConfigureServices 是同一种类型，并且他们的执行顺序和他们的注册顺序保持一致。

执行顺序如下：

先看一下CreateDefaultBuilder:

public static IHostBuilder CreateDefaultBuilder(string[] args)
{
	var builder = new HostBuilder();

	builder.UseContentRoot(Directory.GetCurrentDirectory());
	builder.ConfigureHostConfiguration(config =>
	{
		config.AddEnvironmentVariables(prefix: "DOTNET_");
		if (args != null)
		{
			config.AddCommandLine(args);
		}
	});

	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() && !string.IsNullOrEmpty(env.ApplicationName))
		{
			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) =>
	{
		var isWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows);

		// IMPORTANT: This needs to be added *before* configuration is loaded, this lets
		// the defaults be overridden by the configuration.
		if (isWindows)
		{
			// Default the EventLogLoggerProvider to warning or above
			logging.AddFilter<EventLogLoggerProvider>(level => level >= LogLevel.Warning);
		}

		logging.AddConfiguration(hostingContext.Configuration.GetSection("Logging"));
		logging.AddConsole();
		logging.AddDebug();
		logging.AddEventSourceLogger();

		if (isWindows)
		{
			// Add the EventLogLoggerProvider on windows machines
			logging.AddEventLog();
		}
	})
	.UseDefaultServiceProvider((context, options) =>
	{
		var isDevelopment = context.HostingEnvironment.IsDevelopment();
		options.ValidateScopes = isDevelopment;
		options.ValidateOnBuild = isDevelopment;
	});

	return builder;
}

那么先看下ConfigureWebDefaults 到底干了什么。

internal static void ConfigureWebDefaults(IWebHostBuilder builder)
{
	builder.ConfigureAppConfiguration((ctx, cb) =>
	{
		if (ctx.HostingEnvironment.IsDevelopment())
		{
			StaticWebAssetsLoader.UseStaticWebAssets(ctx.HostingEnvironment, ctx.Configuration);
		}
	});
	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>();

		if (string.Equals("true", hostingContext.Configuration["ForwardedHeaders_Enabled"], StringComparison.OrdinalIgnoreCase))
		{
			services.Configure<ForwardedHeadersOptions>(options =>
			{
				options.ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto;
				// Only loopback proxies are allowed by default. Clear that restriction because forwarders are
				// being enabled by explicit configuration.
				options.KnownNetworks.Clear();
				options.KnownProxies.Clear();
			});

			services.AddTransient<IStartupFilter, ForwardedHeadersStartupFilter>();
		}

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

ConfigureWebDefaults 配置了一些必要的组件。

CreateDefaultBuilder 和 ConfigureWebDefaults 两者可以看到其实就是做一些默认配置让我们的服务能够启动起来。

而后续的配置也就是注册的顺序执行，只是我们的执行顺序晚点罢了。

那么再整理一下，也就是说从原理的执行顺序来说是这样的。

ConfigureHostConfiguration

ConfigureAppConfiguration

ConfigureServices

ConfigureLogging

Configure

以上都是推论。那么直接看源码,因为是构建者模式，所以直接看build。

public IHost Build()
{
	if (_hostBuilt)
	{
		throw new InvalidOperationException("Build can only be called once.");
	}
	_hostBuilt = true;

	BuildHostConfiguration();
	CreateHostingEnvironment();
	CreateHostBuilderContext();
	BuildAppConfiguration();
	CreateServiceProvider();

	return _appServices.GetRequiredService<IHost>();
}

上面的顺序和猜想是一致的。

这里看下BuildHostConfiguration:

private List<Action<IConfigurationBuilder>> _configureHostConfigActions = new List<Action<IConfigurationBuilder>>();
private void BuildHostConfiguration()
{
	var configBuilder = new ConfigurationBuilder()
		.AddInMemoryCollection(); // Make sure there's some default storage since there are no default providers

	foreach (var buildAction in _configureHostConfigActions)
	{
		buildAction(configBuilder);
	}
	_hostConfiguration = configBuilder.Build();
}

那么其实我们按照这种配置，那么其实我们的ConfigureHostConfiguration全部方法都是在_configureHostConfigActions 中，到了builder 的时候就按照我们的注册顺序执行。

那么这里就能解释ConfigureWebHostDefaults 不管顺序都是在最前面，而里面startup的顺序确不一样。

代码解释一波:

public static IHostBuilder ConfigureWebHostDefaults(this IHostBuilder builder, Action<IWebHostBuilder> configure)
{
	return builder.ConfigureWebHost(webHostBuilder =>
	{
		WebHost.ConfigureWebDefaults(webHostBuilder);

		configure(webHostBuilder);
	});
}

ConfigureWebHostDefaults 立即执行，那么log自然就先打印出来了，而其他几个都是延迟执行的。他们的执行顺序和注册顺序有关。

最后再解释一下，webBuilder.UseStartup(); 到底做了什么?

public static IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType)
{
	var startupAssemblyName = startupType.GetTypeInfo().Assembly.GetName().Name;

	hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, startupAssemblyName);

	// Light up the GenericWebHostBuilder implementation
	if (hostBuilder is ISupportsStartup supportsStartup)
	{
		return supportsStartup.UseStartup(startupType);
	}

	return hostBuilder
		.ConfigureServices(services =>
		{
			if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo()))
			{
				services.AddSingleton(typeof(IStartup), startupType);
			}
			else
			{
				services.AddSingleton(typeof(IStartup), sp =>
				{
					var hostingEnvironment = sp.GetRequiredService<IHostEnvironment>();
					return new ConventionBasedStartup(StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName));
				});
			}
		});
}

解释一下IsAssignableFrom:

bool res = {TypeA}.IsAssignableFrom({TypeB}) ;

如果TypeA和TypeB类型一样则返回true；

如果TypeA是TypeB的父类则返回true;

如果TypeB实现了接口TypeA则返回true;

可以看到这里做的是一个IStartup 到 Startup的一个依赖注入，且是单例模式，所以肯定的是Startup 里面的方法会在ConfigureHostConfiguration、ConfigureAppConfiguration之后执行，因为其在ConfigureServices 才开始注入依赖的。

结

因为是应用篇，单纯是一些应用需要知道的，所以比较粗糙，具体的详细在原理篇介绍了。上述只是个人的理解，如果错误，望请指点，谢谢。