Minimal API僅僅是在基于IHost/IHostBuilder的服務承載系統上作了小小的封裝而已，它利用WebApplication和WebApplicationBuilder這兩個類型提供了更加簡潔的API，同時提供了與現有API的兼容。[本文節選《ASP.NET Core 6框架揭秘》第17章]

一、基礎模型
二、WebApplication
三、WebApplication的構建
???? 1. BootstrapHostBuilder
???? 2. ConfigureHostBuilder
???? 3. ConfigureWebHostBuilder
???? 4. WebApplicationBuilder
四、 工廠方法

一、基礎模型

對于由WebApplication和WebApplicationBuilder構建的承載模型，我們沒有必要了解其實現的每一個細節，知道其大致的設計和實現原理就可以了，所以本節會采用最簡潔的代碼模擬這兩個類型的實現。如圖1所示，代表承載應用的WebApplication對象是對一個IHost對象的封裝，而且該類型自身也實現了IHost接口， WebApplication對象其實還是作為一個IHost對象被啟動的。作為構建這的WebApplicationBuilder則是對一個IHostBuilder對象的封裝，它對WebApplication對象的構建體現在利用封裝的IHostBuilder對象構建一個對應的IHost對象，最終利用后者將WebApplication對象創建出來。

圖17-8　完整的請求處理管道

二、WebApplication

WebApplication類型不僅僅實現了IHost接口，還同時實現IApplicationBuilder接口，所以中間件可以直接注冊到這個對象上的。該類型還實現了IEndpointRouteBuilder接口，所以我們還能利用它進行路由注冊，我們在20章才會涉及到路由，所以我們現在先忽略針對該接口的實現。下面的代碼模擬WebApplication類型的實現。如代碼片段所示，WebApplication的構造函數定義了一個IHost類型的參數，它利用這個對象完成了對IHost接口所有成員的實現，針對IApplicationBuilder接口成員的實現則利用創建的ApplicationBuilder對象來完成。WebApplication還提供了一個BuildRequestDelegate方法利用這個ApplicationBuilder對象完成了對中間件管道的構建。

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額外定義了如下的RunAsync和Run方法，它們分別以異步和同步方式啟動承載的應用。調用這兩個方法的時候可以指定監聽地址，指定的地址被添加到IServerAddressesFeature特性中，而服務器正式利用這個特性來提供監聽地址的。

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);}}...
}

三、WebApplication的構建

要創建一個WebApplication對象，只需要提供一個對應的IHost對象即可。IHost對象是通過IHostBuilder對象構建的，所以WebApplicationBuilder需要一個IHostBuilder對象，具體來說是一個HostBuilder對象。我們針對WebApplicationBuilder對象所作的一切設置最終都需要轉移到這個HostBuilder對象上才能生效。為了提供更加簡潔的API，WebApplicationBuilder類型提供了一系列的屬性。比如它利用Serrvices屬性提供了可以直接進行服務注冊的IServiceCollection集合，利用Environment屬性提供了表示當前承載環境的IWebHostEnvironment對象，利用Configuration屬性提供的ConfigurationManager對象不僅可以作為IConfigurationBuilder對象幫助我們完成對配置系統的一切設置，它自身也可以作為IConfiguration對象為我們提供配置。

WebApplicationBuilder還定義了Host和WebHost屬性，對應類型為ConfigureHostBuilder和ConfigureWebHostBuilder，它們分別實現了IHostBuilder和IWebHostBuilder接口，其目的是為了復用IHostBuilder和IWebHostBuilder接口承載的API（主要是擴展方法）。為了會盡可能使用現有方法對IHostBuilder對象進行初始化設置，它還使用了一個實現了IHostBuilder接口的BootstrapHostBuilder類型。有這些對象組成了WebApplicationBuilder針對HostBuilder的構建模型。如圖2所示，WebApplicationBuilder的所有工作都是為了構建它封裝的HostBuilder對象。

當WebApplicationBuilder初始化的時候，它除了會創建這個HostBuilder對象，還會創建存儲服務注冊的IServiceCollection對象，以及用來對配置進行設置的ConfigurationManager對象。接下來它會創建一個BootstrapHostBuilder對象，并將它參數調用相應的方法（比如ConfigureWebHostDefaults方法）將初始化設置收集起來，收集的服務注冊和針對配置系統的設置分別轉移到創建的IServiceCollection和ConfigurationManager對象中，其他設置直接應用到封裝的HostBuilder對象上。

圖2 HostBuilder構建模型

WebApplicationBuilder在此之后會創建出代表承載環境的IWebHostEnvironment對象，并對Environment屬性進行初始化。在得到表示承載上下文的WebHostBuilderContext對象之后，上述的ConfigureHostBuilder和ConfigureWebHostBuilder對象被創建出來，并賦值給Host和WebHost屬性。與BootstrapHostBuilder作用類似，我們利用這兩個對象所作的設置最終都會轉移到上述的三個對象中。當WebApplicationBuilder進行WebApplication對象構建的時候，IServiceCollection對象存儲的服務注冊和ConfigurationManager對象承載配置最終轉移到HostBuilder對象上。此時再利用后者構建出對應的IHost對象，代表承載應用的WebApplication對象最終由該對象構建出來。

1. BootstrapHostBuilder

如下所示的是我們模擬的BootstrapHostBuilder類型的定義。正如上面所說，這個它的作用是收集初始化IHostBuilder對象提供的設置并將它們分別應用到指定的IServiceCollection、ConfigurationManager和IHostBuilder對象上。這一使命體現在BootstrapHostBuilder的Apply方法上，該方法還通過一個輸出參數返回創建的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){//?初始化針對宿主的配置var?hostConfiguration?=?new?ConfigurationManager();_configureHostConfigurations.ForEach(?it?=>?it(hostConfiguration));//?創建承載環境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);//?創建HostBuilderContext上下文var?hostContext?=?new?HostBuilderContext(Properties){Configuration?=?hostConfiguration,HostingEnvironment?=?environment,};//?將針對宿主的配置添加到ConfigurationManager中configuration.AddConfiguration(hostConfiguration,?true);//?初始化針對應用的配置_configureAppConfigurations.ForEach(?it?=>?it(hostContext,?configuration));//?收集服務注冊_configureServices.ForEach(?it?=>?it(hostContext,?services));//?將針對依賴注入容器的設置應用到指定的IHostBuilder對象上_others.ForEach(it?=>?it(hostBuilder));//?將自定義屬性轉移到指定的IHostBuilder對象上foreach?(var?kv?in?Properties){hostBuilder.Properties[kv.Key]?=?kv.Value;}builderContext?=?hostContext;}
}

除了Build方法，IHostBuilder接口中定義的所有方法的參數都是委托，所以實現的這些方法將提供的委托收集起來。在Apply方法中，我們通過執行這些委托對象，將初始化設置應用到指定的IServiceCollection、ConfigurationManager和IHostBuilder對象上，并根據初始化宿主配置構建出代表承載環境的HostingEnvironment對象。該方法最后根據承載環境結合配置將HostBuilderContext上下文創建出來，并以輸出參數的形式返回。

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));
}

2. ConfigureHostBuilder

ConfigureHostBuilder是在應用了BootstrapHostBuilder收集的初始化設置之后創建的，在創建該對象時提供了HostBuilderContext上下文， ConfigurationManager和IServiceCollection對象。提供的服務注冊直接添加到IServiceCollection對象中，針對配置的設置已經應用到ConfigurationManager對象，直接針對IHostBuilder對象的設置則利用_configureActions字段暫存起來。

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);//?與環境相關的三個配置不允許改變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對象一旦被創建出來后，針對承載環境的配置是不能改變的，所以ConfigureHostBuilder的ConfigureHostConfiguration方法針對此添加了相應的驗證。兩個UseServiceProviderFactory方法和ConfigureContainer方法針對依賴注入容器的設置最終需要應用到IHostBuilder對象上，所以我們將方法中提供的委托對象利用configureActions字段存起來，并最終利用Apply方法應用到指定的IHostBuilder對象上。

3. ConfigureWebHostBuilder

ConfigureWebHostBuilder同樣是在應用了BootstrapHostBuilder提供的初始化設置后創建的，創建該對象時能夠提供WebHostBuilderContext上下文和承載配置和服務注冊的ConfigurationManager和IServiceCollection對象。由于IWebHostBuilder接口定義的方法只涉及服務注冊和針對配置的設置，所以方法提供的委托對象可以直接應用到這兩個對象上。

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;}
}

我們在前面說過，傳統承載方式將初始化操作定義在注冊的Startup類型的編程方式在Minima API中已經不再被支持了，所以WebApplicationBuilder本不該實現ISupportsStartup接口，但是我們希望用戶在采用這種編程方式時得到顯式的提醒，所以依然讓它實現該接口，并在實現的方法中拋出NotImplementedException類型的異常。

4. WebApplicationBuilder

如下的代碼片段模擬了WebApplicationBuilder針對WebApplication的構建。它的構造函數會創建一個BootstrapHostBuilder對象，調用它的ConfigureDefaults和ConfigureWebHostDefaults擴展方法將初始化設置收集起來。ConfigureWebHostDefaults方法會利用提供的Action<IWebHostBuilder>委托進行中間件的注冊，由于中間件的注冊被轉移到WebApplication對象上，并且它提供了一個BuildRequestDelegate方法返回由注冊中間件組成的管道，所以在這里只需調用構建的WebApplication對象（通過_application字段表示，雖然此時尚未創建，但是中間件真正被注冊時會被創建出來）的這個方法，并將返回的RequestDelegate對象作為參數調用IApplicationBuilder接口的Run方法將中間件管道注冊為請求處理器。

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){//創建BootstrapHostBuilder并利用它收集初始化過程中設置的配置、服務和針對依賴注入容器的設置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);}//?將WebApplicationOptions配置選項轉移到配置中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);}});//?將BootstrapHostBuilder收集到配置和服務轉移到Configuration和Services上//?將應用到BootstrapHostBuilder上針對依賴注入溶質的設置轉移到_hostBuilder上//?得到BuilderContext上下文bootstrap.Apply(_hostBuilder,?Configuration,??Services,?out?var?builderContext);//?如果提供了命令行參數，在Configuration上添加對應配置源if?(options.Args?.Any()?==?true){Configuration.AddCommandLine(options.Args);}//?構建WebHostBuilderContext上下文//?初始化Host、WebHost和Logging屬性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(){//?將ConfigurationManager的配置轉移到_hostBuilder_hostBuilder.ConfigureAppConfiguration(builder?=>{builder.AddConfiguration(Configuration);foreach?(var?kv?in??((IConfigurationBuilder)Configuration).Properties){builder.Properties[kv.Key]?=?kv.Value;}});//?將添加的服務注冊轉移到_hostBuilder_hostBuilder.ConfigureServices((_,?services)?=>{foreach?(var?service?in?Services){services.Add(service);}});//?將應用到Host屬性上的設置轉移到_hostBuilderHost.Apply(_hostBuilder);//?利用_hostBuilder構建的IHost對象創建WebApplicationreturn?_application?=?new?WebApplication(_hostBuilder.Build());}
}

接下來BootstrapHostBuilder的ConfigureHostConfiguration方法被調用，我們利用它將提供的WebApplicationOptions配置選項轉移到BootstrapHostBuilder針對宿主的配置上。針對IHostBuilder初始化設置應用到BootstrapHostBuilder對象上之后，我們調用其Apply方法將這些設置分別轉移到承載服務注冊和配置的IServiceCollection和ConfigurationManager對象，以及封裝的HostBuilder對象上。Apply方法利用輸出參數提供了HostBuilderContext上下文，我們進一步從中提取出WebHostBuilderContext上下文（GenericWebHostBuilder會將構建的WebHostBuilderContext上下文置于HostBuilderContext對象的屬性字典中）。我們利用這個上下文將ConfigureHostBuilder和ConfigureWebHostBuilder對象創建出來，并作為Host和WebHost屬性。用于對日志做進一步設置的Logging屬性也在這里被初始化，返回的LoggingBuilder對象僅僅是對IServiceCollection對象的簡單封裝而已。

構建WebApplication對象的Build方法分別調用ConfigureAppConfiguration和ConfigureServices方法將ConfigurationManager和IServiceCollection對象承載的配置和服務注冊轉移到HostBuilder對象上。它接下來提取出Host屬性返回的ConfigureHostBuilder對象，并調用其Apply方法將應用在該對象上針對依賴注入容器的設置轉移到HostBuilder對象上。至此所有的設置全部轉移到了HostBuilder對象上，我們調用其Build方法構建出對應的IHost對象后，最后利用后者將代碼承載應用的WebApplication對象構建出來。我們將這個對象賦值到_application字段上，前面調用ConfigureWebHostDefaults擴展方法提供的委托會將它的BuildRequestDelegate方法構建的中間件管道作為請求處理器。

四、 工廠方法

代表承載應用的WebApplication對象由WebApplicationBuilder構建的，但是我們一般不會通過調用構造函數的方式來創建WebApplicationBuilder對象，這有違“面向接口”編程的原則，所以我們都會使用WebApplication類型提供的靜態工廠方法來創建它。WebApplication除了提供了三個用于創建WebApplicationBuilder的CreateBuilder方法重載，還提供了一個直接創建WebApplication對象的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();}
}

本節內容通過針對WebApplication和WebApplicationBuilder這兩個類型的實現模擬來講解Minimal API的實現原理。一方面為了讓講解更加清晰，另一方面也出于篇幅的限制，不得不省去很多細枝末節的內容，但是設計思想和實現原理別無二致。上面提供的源代碼也不是偽代碼，如下所示的就是在“模擬的Minimal API”構建的ASP.NET Core應用，它是可以正常運行的。如果讀者朋友們對真實的實現感興趣，可以將它作為一個“向導”去探尋“真實的Minimal API”。

var?app?=?App.WebApplication.Create();
app.Run(httpContext?=>?httpContext.Response.WriteAsync("Hello?World!"));
app.Run();