Minimal API仅仅是在基于IHost/IHostBuilder的服务承载系统上作了小小的封装而已，它利用WebApplication和WebApplicationBuilder这两个类型提供了更加简洁的API，同时提供了与现有API的兼容。要成分理解Minimal API的实现原理 Minimal API仅仅是在基于IHost/IHostBuilder的服务承载系统上作了小小的封装而已，它利用WebApplication和WebApplicationBuilder这两个类型提供了更加简洁的API，同时提供了与现有API的兼容。要成分理解Minimal API的实现原理，得先对服务承载系统有基本的理解，对此不了解的可以参阅《服务承载模型[上篇]》、《服务承载模型[下篇]》、《承载服务启动流程[上篇]》和《承载服务启动流程[下篇]》。对于本篇提供的模拟代码，可以从这里下载。
一、基础模型
二、WebApplication
三、WebApplication的构建
1. BootstrapHostBuilder
2. ConfigureHostBuilder
3. ConfigureWebHostBuilder
4. WebApplicationBuilder
四、 工厂方法
对于由WebApplication和WebApplicationBuilder构建的承载模型，我们没有必要了解其实现的每一个细节，知道其大致的设计和实现原理就可以了，所以本节会采用最简洁的代码模拟这两个类型的实现。如图1所示，代表承载应用的WebApplication对象是对一个IHost对象的封装，而且该类型自身也实现了IHost接口， WebApplication对象其实还是作为一个IHost对象被启动的。作为构建这的WebApplicationBuilder则是对一个IHostBuilder对象的封装，它对WebApplication对象的构建体现在利用封装的IHostBuilder对象构建一个对应的IHost对象，最终利用后者将WebApplication对象创建出来。
图17-8 完整的请求处理管道
WebApplication类型不仅仅实现了IHost接口，还同时实现IApplicationBuilder接口，所以中间件可以直接注册到这个对象上的。该类型还实现了IEndpointRouteBuilder接口，所以我们还能利用它进行路由注册，我们在20章才会涉及到路由，所以我们现在先忽略针对该接口的实现。下面的代码模拟WebApplication类型的实现。如代码片段所示，WebApplication的构造函数定义了一个IHost类型的参数，它利用这个对象完成了对IHost接口所有成员的实现，针对IApplicationBuilder接口成员的实现则利用创建的ApplicationBuilder对象来完成。WebApplication还提供了一个BuildRequestDelegate方法利用这个ApplicationBuilder对象完成了对中间件管道的构建。
public class WebApplication : IApplicationBuilder, IHost{ private readonly IHost _host; private readonly ApplicationBuilder _app; public WebApplication(IHost host) { _host = host; _app = new ApplicationBuilder(host.Services); } IServiceProvider IHost.Services => _host.Services; Task IHost.StartAsync(CancellationToken cancellationToken) => _host.StartAsync(cancellationToken); Task IHost.StopAsync(CancellationToken cancellationToken) => _host.StopAsync(cancellationToken); IServiceProvider IApplicationBuilder.ApplicationServices { get => _app.ApplicationServices; set => _app.ApplicationServices = value; } IFeatureCollection IApplicationBuilder.ServerFeatures => _app.ServerFeatures; IDictionary<string, object?> IApplicationBuilder.Properties => _app.Properties; RequestDelegate IApplicationBuilder.Build() => _app.Build(); IApplicationBuilder IApplicationBuilder.New() => _app.New(); IApplicationBuilder IApplicationBuilder.Use(Func<RequestDelegate, RequestDelegate> middleware) => _app.Use(middleware); void IDisposable.Dispose() => _host.Dispose(); public IServiceProvider Services => _host.Services; internal RequestDelegate BuildRequestDelegate() => _app.Build(); ...}
WebApplication额外定义了如下的RunAsync和Run方法，它们分别以异步和同步方式启动承载的应用。调用这两个方法的时候可以指定监听地址，指定的地址被添加到IServerAddressesFeature特性中，而服务器正式利用这个特性来提供监听地址的。
public class WebApplication : IApplicationBuilder, IHost{ private readonly IHost _host; public ICollection<string> Urls => _host.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()?.Addresses ?? throw new InvalidOperationException("IServerAddressesFeature is not found."); public Task RunAsync(string? url = null) { Listen(url); return HostingAbstractionsHostExtensions.RunAsync(this); } public void Run(string? url = null) { Listen(url); HostingAbstractionsHostExtensions.Run(this); } private void Listen(string? url) { if (url is not null) { var addresses = _host.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()?.Addresses ?? throw new InvalidOperationException("IServerAddressesFeature is not found."); addresses.Clear(); addresses.Add(url); } } ...}
要创建一个WebApplication对象，只需要提供一个对应的IHost对象即可。IHost对象是通过IHostBuilder对象构建的，所以WebApplicationBuilder需要一个IHostBuilder对象，具体来说是一个HostBuilder对象。我们针对WebApplicationBuilder对象所作的一切设置最终都需要转移到这个HostBuilder对象上才能生效。为了提供更加简洁的API，WebApplicationBuilder类型提供了一系列的属性。比如它利用Serrvices属性提供了可以直接进行服务注册的IServiceCollection集合，利用Environment属性提供了表示当前承载环境的IWebHostEnvironment对象，利用Configuration属性提供的ConfigurationManager对象不仅可以作为IConfigurationBuilder对象帮助我们完成对配置系统的一切设置，它自身也可以作为IConfiguration对象为我们提供配置。
WebApplicationBuilder还定义了Host和WebHost属性，对应类型为ConfigureHostBuilder和ConfigureWebHostBuilder，它们分别实现了IHostBuilder和IWebHostBuilder接口，其目的是为了复用IHostBuilder和IWebHostBuilder接口承载的API（主要是扩展方法）。为了会尽可能使用现有方法对IHostBuilder对象进行初始化设置，它还使用了一个实现了IHostBuilder接口的BootstrapHostBuilder类型。有这些对象组成了WebApplicationBuilder针对HostBuilder的构建模型。如图2所示，WebApplicationBuilder的所有工作都是为了构建它封装的HostBuilder对象。
当WebApplicationBuilder初始化的时候，它除了会创建这个HostBuilder对象，还会创建存储服务注册的IServiceCollection对象，以及用来对配置进行设置的ConfigurationManager对象。接下来它会创建一个BootstrapHostBuilder对象，并将它参数调用相应的方法（比如ConfigureWebHostDefaults方法）将初始化设置收集起来，收集的服务注册和针对配置系统的设置分别转移到创建的IServiceCollection和ConfigurationManager对象中，其他设置直接应用到封装的HostBuilder对象上。
图2 HostBuilder构建模型
WebApplicationBuilder在此之后会创建出代表承载环境的IWebHostEnvironment对象，并对Environment属性进行初始化。在得到表示承载上下文的WebHostBuilderContext对象之后，上述的ConfigureHostBuilder和ConfigureWebHostBuilder对象被创建出来，并赋值给Host和WebHost属性。与BootstrapHostBuilder作用类似，我们利用这两个对象所作的设置最终都会转移到上述的三个对象中。当WebApplicationBuilder进行WebApplication对象构建的时候，IServiceCollection对象存储的服务注册和ConfigurationManager对象承载配置最终转移到HostBuilder对象上。此时再利用后者构建出对应的IHost对象，代表承载应用的WebApplication对象最终由该对象构建出来。
如下所示的是我们模拟的BootstrapHostBuilder类型的定义。正如上面所说，这个它的作用是收集初始化IHostBuilder对象提供的设置并将它们分别应用到指定的IServiceCollection、ConfigurationManager和IHostBuilder对象上。这一使命体现在BootstrapHostBuilder的Apply方法上，该方法还通过一个输出参数返回创建的HostBuilderContext上下文。
public class BootstrapHostBuilder : IHostBuilder{ private readonly List<Action<IConfigurationBuilder>> _configureHostConfigurations = new(); private readonly List<Action<HostBuilderContext, IConfigurationBuilder>> _configureAppConfigurations = new(); private readonly List<Action<HostBuilderContext, IServiceCollection>> _configureServices = new(); private readonly List<Action<IHostBuilder>> _others = new(); public IDictionary<object, object> Properties { get; } = new Dictionary<object, object>(); public IHost Build() => throw new NotImplementedException(); public IHostBuilder ConfigureHostConfiguration(Action<IConfigurationBuilder> configureDelegate) { _configureHostConfigurations.Add(configureDelegate); return this; } public IHostBuilder ConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate) { _configureAppConfigurations.Add(configureDelegate); return this; } public IHostBuilder ConfigureServices(Action<HostBuilderContext, IServiceCollection> configureDelegate) { _configureServices.Add(configureDelegate); return this; } public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(IServiceProviderFactory<TContainerBuilder> factory) { _others.Add(builder => builder.UseServiceProviderFactory(factory)); return this; } public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(Func<HostBuilderContext, IServiceProviderFactory<TContainerBuilder>> factory) { _others.Add(builder => builder.UseServiceProviderFactory(factory)); return this; } public IHostBuilder ConfigureContainer<TContainerBuilder>(Action<HostBuilderContext, TContainerBuilder> configureDelegate) { _others.Add(builder => builder.ConfigureContainer(configureDelegate)); return this; } internal void Apply(IHostBuilder hostBuilder, ConfigurationManager configuration, IServiceCollection services, out HostBuilderContext builderContext) { // 初始化针对宿主的配置 var hostConfiguration = new ConfigurationManager(); _configureHostConfigurations.ForEach(it => it(hostConfiguration)); // 创建承载环境 var environment = new HostingEnvironment() { ApplicationName = hostConfiguration[HostDefaults.ApplicationKey], EnvironmentName = hostConfiguration[HostDefaults.EnvironmentKey] ?? Environments.Production, ContentRootPath = HostingPathResolver.ResolvePath(hostConfiguration[HostDefaults.ContentRootKey]) }; environment.ContentRootFileProvider = new PhysicalFileProvider(environment.ContentRootPath); // 创建HostBuilderContext上下文 var hostContext = new HostBuilderContext(Properties) { Configuration = hostConfiguration, HostingEnvironment = environment, }; // 将针对宿主的配置添加到ConfigurationManager中 configuration.AddConfiguration(hostConfiguration, true); // 初始化针对应用的配置 _configureAppConfigurations.ForEach(it => it(hostContext, configuration)); // 收集服务注册 _configureServices.ForEach(it => it(hostContext, services)); // 将针对依赖注入容器的设置应用到指定的IHostBuilder对象上 _others.ForEach(it => it(hostBuilder)); // 将自定义属性转移到指定的IHostBuilder对象上 foreach (var kv in Properties) { hostBuilder.Properties[kv.Key] = kv.Value; } builderContext = hostContext; }}
除了Build方法，IHostBuilder接口中定义的所有方法的参数都是委托，所以实现的这些方法将提供的委托收集起来。在Apply方法中，我们通过执行这些委托对象，将初始化设置应用到指定的IServiceCollection、ConfigurationManager和IHostBuilder对象上，并根据初始化宿主配置构建出代表承载环境的HostingEnvironment对象。该方法最后根据承载环境结合配置将HostBuilderContext上下文创建出来，并以输出参数的形式返回。
internal static class HostingPathResolver{ public static string ResolvePath(string? contentRootPath) => ResolvePath(contentRootPath, .BaseDirectory); public static string ResolvePath(string? contentRootPath, string basePath) => string.IsNullOrEmpty(contentRootPath) ? Path.GetFullPath(basePath): Path.IsPathRooted(contentRootPath)? Path.GetFullPath(contentRootPath) : Path.GetFullPath(Path.Combine(Path.GetFullPath(basePath), contentRootPath));}
ConfigureHostBuilder是在应用了BootstrapHostBuilder收集的初始化设置之后创建的，在创建该对象时提供了HostBuilderContext上下文， ConfigurationManager和IServiceCollection对象。提供的服务注册直接添加到IServiceCollection对象中，针对配置的设置已经应用到ConfigurationManager对象，直接针对IHostBuilder对象的设置则利用_configureActions字段暂存起来。
public class ConfigureHostBuilder : IHostBuilder{ private readonly ConfigurationManager _configuration; private readonly IServiceCollection _services; private readonly HostBuilderContext _context; private readonly List<Action<IHostBuilder>> _configureActions = new(); internal ConfigureHostBuilder(HostBuilderContext context, ConfigurationManager configuration, IServiceCollection services) { _configuration = configuration; _services = services; _context = context; } public IDictionary<object, object> Properties => _context.Properties; public IHost Build() => throw new NotImplementedException(); public IHostBuilder ConfigureAppConfiguration(Action<HostBuilderContext, IConfigurationBuilder> configureDelegate) => Configure(() => configureDelegate(_context, _configuration)); public IHostBuilder ConfigureHostConfiguration(Action<IConfigurationBuilder> configureDelegate) { var applicationName = _configuration[HostDefaults.ApplicationKey]; var contentRoot = _context.HostingEnvironment.ContentRootPath; var environment = _configuration[HostDefaults.EnvironmentKey]; configureDelegate(_configuration); // 与环境相关的三个配置不允许改变 Validate(applicationName, HostDefaults.ApplicationKey, "Application name cannot be changed."); Validate(contentRoot, HostDefaults.ContentRootKey, "Content root cannot be changed."); Validate(environment, HostDefaults.EnvironmentKey, "Environment name cannot be changed."); return this; void Validate(string previousValue, string key, string message) { if (!string.Equals(previousValue, _configuration[key], StringComparison.OrdinalIgnoreCase)) { throw new NotSupportedException(message); } } } public IHostBuilder ConfigureServices(Action<HostBuilderContext, IServiceCollection> configureDelegate) => Configure(() => configureDelegate(_context, _services)); public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(IServiceProviderFactory<TContainerBuilder> factory) => Configure(() => _configureActions.Add(b => b.UseServiceProviderFactory(factory))); public IHostBuilder UseServiceProviderFactory<TContainerBuilder>(Func<HostBuilderContext, IServiceProviderFactory<TContainerBuilder>> factory) => Configure(() => _configureActions.Add(b => b.UseServiceProviderFactory(factory))); public IHostBuilder ConfigureContainer<TContainerBuilder>(Action<HostBuilderContext, TContainerBuilder> configureDelegate) => Configure(() => _configureActions.Add(b => b.ConfigureContainer(configureDelegate))); private IHostBuilder Configure(Action configure) { configure(); return this; } internal void Apply(IHostBuilder hostBuilder) => _configureActions.ForEach(op => op(hostBuilder));}
WebApplicationBuilder对象一旦被创建出来后，针对承载环境的配置是不能改变的，所以ConfigureHostBuilder的ConfigureHostConfiguration方法针对此添加了相应的验证。两个UseServiceProviderFactory方法和ConfigureContainer方法针对依赖注入容器的设置最终需要应用到IHostBuilder对象上，所以我们将方法中提供的委托对象利用configureActions字段存起来，并最终利用Apply方法应用到指定的IHostBuilder对象上。
ConfigureWebHostBuilder同样是在应用了BootstrapHostBuilder提供的初始化设置后创建的，创建该对象时能够提供WebHostBuilderContext上下文和承载配置和服务注册的ConfigurationManager和IServiceCollection对象。由于IWebHostBuilder接口定义的方法只涉及服务注册和针对配置的设置，所以方法提供的委托对象可以直接应用到这两个对象上。
public class ConfigureWebHostBuilder : IWebHostBuilder, ISupportsStartup{ private readonly WebHostBuilderContext _builderContext; private readonly IServiceCollection _services; private readonly ConfigurationManager _configuration; public ConfigureWebHostBuilder(WebHostBuilderContext builderContext, ConfigurationManager configuration, IServiceCollection services) { _builderContext = builderContext; _services = services; _configuration = configuration; } public IWebHost Build() => throw new NotImplementedException(); public IWebHostBuilder ConfigureAppConfiguration(Action<WebHostBuilderContext, IConfigurationBuilder> configureDelegate) => Configure(() => configureDelegate(_builderContext, _configuration)); public IWebHostBuilder ConfigureServices(Action<IServiceCollection> configureServices) => Configure(() => configureServices(_services)); public IWebHostBuilder ConfigureServices(Action<WebHostBuilderContext, IServiceCollection> configureServices) => Configure(() => configureServices(_builderContext, _services)); public string? GetSetting(string key) => _configuration[key]; public IWebHostBuilder UseSetting(string key, string? value) => Configure(() => _configuration[key] = value); IWebHostBuilder ISupportsStartup.UseStartup(Type startupType) => throw new NotImplementedException(); IWebHostBuilder ISupportsStartup.UseStartup<TStartup>(Func<WebHostBuilderContext, TStartup> startupFactory) => throw new NotImplementedException(); IWebHostBuilder ISupportsStartup.Configure(Action<IApplicationBuilder> configure) => throw new NotImplementedException(); IWebHostBuilder ISupportsStartup.Configure(Action<WebHostBuilderContext, IApplicationBuilder> configure) => throw new NotImplementedException(); private IWebHostBuilder Configure(Action configure) { configure(); return this; }}
我们在前面说过，传统承载方式将初始化操作定义在注册的Startup类型的编程方式在Minima API中已经不再被支持了，所以WebApplicationBuilder本不该实现ISupportsStartup接口，但是我们希望用户在采用这种编程方式时得到显式的提醒，所以依然让它实现该接口，并在实现的方法中抛出NotImplementedException类型的异常。
如下的代码片段模拟了WebApplicationBuilder针对WebApplication的构建。它的构造函数会创建一个BootstrapHostBuilder对象，调用它的ConfigureDefaults和ConfigureWebHostDefaults扩展方法将初始化设置收集起来。ConfigureWebHostDefaults方法会利用提供的Action<IWebHostBuilder>委托进行中间件的注册，由于中间件的注册被转移到WebApplication对象上，并且它提供了一个BuildRequestDelegate方法返回由注册中间件组成的管道，所以在这里只需调用构建的WebApplication对象（通过_application字段表示，虽然此时尚未创建，但是中间件真正被注册时会被创建出来）的这个方法，并将返回的RequestDelegate对象作为参数调用IApplicationBuilder接口的Run方法将中间件管道注册为请求处理器。
public class WebApplicationBuilder{ private readonly HostBuilder _hostBuilder = new HostBuilder(); private WebApplication _application; public ConfigurationManager Configuration { get; } = new ConfigurationManager(); public IServiceCollection Services { get; } = new ServiceCollection(); public IWebHostEnvironment Environment { get; } public ConfigureHostBuilder Host { get; } public ConfigureWebHostBuilder WebHost { get; } public ILoggingBuilder Logging { get; } public WebApplicationBuilder(WebApplicationOptions options) { //创建BootstrapHostBuilder并利用它收集初始化过程中设置的配置、服务和针对依赖注入容器的设置 var args = options.Args; var bootstrap = new BootstrapHostBuilder(); bootstrap .ConfigureDefaults(null) .ConfigureWebHostDefaults(webHostBuilder => webHostBuilder.Configure(app => app.Run(_application.BuildRequestDelegate()))) .ConfigureHostConfiguration(config => { // 添加命令行配置源 if (args?.Any() == true) { config.AddCommandLine(args); } // 将WebApplicationOptions配置选项转移到配置中 Dictionary<string, string>? settings = null; if (options.EnvironmentName is not null) (settings ??= new())[HostDefaults.EnvironmentKey] = options.EnvironmentName; if (options.ApplicationName is not null) (settings ??= new())[HostDefaults.ApplicationKey] = options.ApplicationName; if (options.ContentRootPath is not null) (settings ??= new())[HostDefaults.ContentRootKey] = options.ContentRootPath; if (options.WebRootPath is not null) (settings ??= new())[WebHostDefaults.WebRootKey] = options.EnvironmentName; if (settings != null) { config.AddInMemoryCollection(settings); } }); // 将BootstrapHostBuilder收集到配置和服务转移到Configuration和Services上 // 将应用到BootstrapHostBuilder上针对依赖注入溶质的设置转移到_hostBuilder上 // 得到BuilderContext上下文 bootstrap.Apply(_hostBuilder, Configuration, Services, out var builderContext); // 如果提供了命令行参数，在Configuration上添加对应配置源 if (options.Args?.Any() == true) { Configuration.AddCommandLine(options.Args); } // 构建WebHostBuilderContext上下文 // 初始化Host、WebHost和Logging属性 var webHostContext = (WebHostBuilderContext)builderContext.Properties[typeof(WebHostBuilderContext)]; Environment = webHostContext.HostingEnvironment; Host = new ConfigureHostBuilder(builderContext, Configuration, Services); WebHost = new ConfigureWebHostBuilder(webHostContext, Configuration, Services); Logging = new LogginigBuilder(Services); } public WebApplication Build() { // 将ConfigurationManager的配置转移到_hostBuilder _hostBuilder.ConfigureAppConfiguration(builder => { builder.AddConfiguration(Configuration); foreach (var kv in ((IConfigurationBuilder)Configuration).Properties) { builder.Properties[kv.Key] = kv.Value; } }); // 将添加的服务注册转移到_hostBuilder _hostBuilder.ConfigureServices((_, services) => { foreach (var service in Services) { services.Add(service); } }); // 将应用到Host属性上的设置转移到_hostBuilder Host.Apply(_hostBuilder); // 利用_hostBuilder构建的IHost对象创建WebApplication return _application = new WebApplication(_hostBuilder.Build()); }}
接下来BootstrapHostBuilder的ConfigureHostConfiguration方法被调用，我们利用它将提供的WebApplicationOptions配置选项转移到BootstrapHostBuilder针对宿主的配置上。针对IHostBuilder初始化设置应用到BootstrapHostBuilder对象上之后，我们调用其Apply方法将这些设置分别转移到承载服务注册和配置的IServiceCollection和ConfigurationManager对象，以及封装的HostBuilder对象上。Apply方法利用输出参数提供了HostBuilderContext上下文，我们进一步从中提取出WebHostBuilderContext上下文（GenericWebHostBuilder会将构建的WebHostBuilderContext上下文置于HostBuilderContext对象的属性字典中）。我们利用这个上下文将ConfigureHostBuilder和ConfigureWebHostBuilder对象创建出来，并作为Host和WebHost属性。用于对日志做进一步设置的Logging属性也在这里被初始化，返回的LoggingBuilder对象仅仅是对IServiceCollection对象的简单封装而已。
构建WebApplication对象的Build方法分别调用ConfigureAppConfiguration和ConfigureServices方法将ConfigurationManager和IServiceCollection对象承载的配置和服务注册转移到HostBuilder对象上。它接下来提取出Host属性返回的ConfigureHostBuilder对象，并调用其Apply方法将应用在该对象上针对依赖注入容器的设置转移到HostBuilder对象上。至此所有的设置全部转移到了HostBuilder对象上，我们调用其Build方法构建出对应的IHost对象后，最后利用后者将代码承载应用的WebApplication对象构建出来。我们将这个对象赋值到_application字段上，前面调用ConfigureWebHostDefaults扩展方法提供的委托会将它的BuildRequestDelegate方法构建的中间件管道作为请求处理器。
代表承载应用的WebApplication对象由WebApplicationBuilder构建的，但是我们一般不会通过调用构造函数的方式来创建WebApplicationBuilder对象，这有违“面向接口”编程的原则，所以我们都会使用WebApplication类型提供的静态工厂方法来创建它。WebApplication除了提供了三个用于创建WebApplicationBuilder的CreateBuilder方法重载，还提供了一个直接创建WebApplication对象的Create方法。
public sealed class WebApplication{ public static WebApplicationBuilder CreateBuilder() => new WebApplicationBuilder(new WebApplicationOptions()); public static WebApplicationBuilder CreateBuilder(string[] args) { var options = new WebApplicationOptions(); options.Args = args; return new WebApplicationBuilder(options); } public static WebApplicationBuilder CreateBuilder(WebApplicationOptions options) => new WebApplicationBuilder(options, null); public static WebApplication Create(string[]? args = null) { var options = new WebApplicationOptions(); options.Args = args; return new WebApplicationBuilder(options).Build(); }}
本节内容通过针对WebApplication和WebApplicationBuilder这两个类型的实现模拟来讲解Minimal API的实现原理。一方面为了让讲解更加清晰，另一方面也出于篇幅的限制，不得不省去很多细枝末节的内容，但是设计思想和实现原理别无二致。上面提供的源代码也不是伪代码，如下所示的就是在“模拟的Minimal API”构建的ASP.NET Core应用，它是可以正常运行的。如果读者朋友们对真实的实现感兴趣，可以将它作为一个“向导”去探寻“真实的Minimal API”。
var app = App.WebApplication.Create();app.Run(httpContext => httpContext.Response.WriteAsync("Hello World!"));app.Run();