ASP.NET Core 6框架揭秘实例演示[27]:ASP.NET Core 6 Minimal API的模拟实现

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

一、基础模型

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。

image

图17-8 完整的请求处理管道

二、WebApplication

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);
        }
    }
    ...
}

三、WebApplication的构建

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。

image

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。

1. BootstrapHostBuilder

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));
}

2. ConfigureHostBuilder

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。

3. ConfigureWebHostBuilder

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。

4. WebApplicationBuilder

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。

四、 工厂方法

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();

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