ASP.NET Core 是一个跨平台的高性能开源框架，是一个用于连接到互联网的基于云的现代应用程序。 ASP.NET Core 用于构建如 Web 应用、物联网（IoT）应用和移动后端应用，这些应用可以在 .NET Core 或 .NET Framework上运行，你可以在 Windows、Mac 和 Linux 上跨平台的开发和运行你的 ASP.NET Core 应用。
ASP.NET Core项目解读
由于相比以前传统的ASP.NET项目而言， ASP.NET Core 的项目结构也变的完全不同，对于刚接触 ASP.NET Core 的小白来讲，我觉得有必要来简单解读一下。
注：这里的解读是针对ASP.NET Core 2.1
项目文件夹总预览
Program类
public class Program { public static void Main(string[] args) { CreateWebHostBuilder(args).Build().Run(); } public static IWebHostBuilder CreateWebHostBuilder(string[] args) => WebHost.CreateDefaultBuilder(args) .UseStartup<Startup>(); }
我们可以看到程序中有一个 Program 类型，按照我们以前的认知，这个类不是只出现在控制台项目中的嘛。那么接下来我们来看看这个类里面都有些什么，又都干了什么。
WebHost
webHost相当于一个用Socket实现的Web服务。
1、WebHost使用了建造者模式
2、CreateDefaultBuilder：我们用反编译工具来查看下这个方法里面都干了什么。
public static IWebHostBuilder CreateDefaultBuilder(string[] args) { IWebHostBuilder arg_31_0 = new WebHostBuilder(); Action<WebHostBuilderContext, KestrelServerOptions> arg_31_1; if ((arg_31_1 = WebHost.<>c.<>9__8_0) == null) { arg_31_1 = (WebHost.<>c.<>9__8_0 = new Action<WebHostBuilderContext, KestrelServerOptions>(WebHost.<>c.<>9.<CreateDefaultBuilder>b__8_0)); } IWebHostBuilder arg_70_0 = HostingAbstractionsWebHostBuilderExtensions.UseContentRoot(WebHostBuilderKestrelExtensions.UseKestrel(arg_31_0, arg_31_1), Directory.GetCurrentDirectory()).ConfigureAppConfiguration(delegate(WebHostBuilderContext hostingContext, IConfigurationBuilder config) { IHostingEnvironment hostingEnvironment = hostingContext.get_HostingEnvironment(); JsonConfigurationExtensions.AddJsonFile(JsonConfigurationExtensions.AddJsonFile(config, "appsettings.json", true, true), string.Format("appsettings.{0}.json", hostingEnvironment.get_EnvironmentName()), true, true); if (HostingEnvironmentExtensions.IsDevelopment(hostingEnvironment)) { Assembly assembly = Assembly.Load(new AssemblyName(hostingEnvironment.get_ApplicationName())); if (assembly != null) { UserSecretsConfigurationExtensions.AddUserSecrets(config, assembly, true); } } EnvironmentVariablesExtensions.AddEnvironmentVariables(config); if (args != null) { CommandLineConfigurationExtensions.AddCommandLine(config, args); } }); Action<WebHostBuilderContext, ILoggingBuilder> arg_70_1; if ((arg_70_1 = WebHost.<>c.<>9__8_2) == null) { arg_70_1 = (WebHost.<>c.<>9__8_2 = new Action<WebHostBuilderContext, ILoggingBuilder>(WebHost.<>c.<>9.<CreateDefaultBuilder>b__8_2)); } IWebHostBuilder arg_94_0 = WebHostBuilderExtensions.ConfigureLogging(arg_70_0, arg_70_1); Action<WebHostBuilderContext, IServiceCollection> arg_94_1; if ((arg_94_1 = WebHost.<>c.<>9__8_3) == null) { arg_94_1 = (WebHost.<>c.<>9__8_3 = new Action<WebHostBuilderContext, IServiceCollection>(WebHost.<>c.<>9.<CreateDefaultBuilder>b__8_3)); } IWebHostBuilder arg_BD_0 = WebHostBuilderIISExtensions.UseIISIntegration(arg_94_0.ConfigureServices(arg_94_1)); Action<WebHostBuilderContext, ServiceProviderOptions> arg_BD_1; if ((arg_BD_1 = WebHost.<>c.<>9__8_4) == null) { arg_BD_1 = (WebHost.<>c.<>9__8_4 = new Action<WebHostBuilderContext, ServiceProviderOptions>(WebHost.<>c.<>9.<CreateDefaultBuilder>b__8_4)); } IWebHostBuilder webHostBuilder = WebHostBuilderExtensions.UseDefaultServiceProvider(arg_BD_0, arg_BD_1); if (args != null) { HostingAbstractionsWebHostBuilderExtensions.UseConfiguration(webHostBuilder, CommandLineConfigurationExtensions.AddCommandLine(new ConfigurationBuilder(), args).Build()); } return webHostBuilder; }
以下是Action<WebHostBuilderContext, KestrelServerOptions> arg_31_1;委托的逻辑
internal void <CreateDefaultBuilder>b__8_0(WebHostBuilderContext builderContext, KestrelServerOptions options) { options.Configure(builderContext.get_Configuration().GetSection("Kestrel")); }
以下是Action<WebHostBuilderContext, IServiceCollection> arg_94_1;委托的逻辑
internal void <CreateDefaultBuilder>b__8_3(WebHostBuilderContext hostingContext, IServiceCollection services) { WebHost.<>c__DisplayClass8_1 <>c__DisplayClass8_ = new WebHost.<>c__DisplayClass8_1(); <>c__DisplayClass8_.hostingContext = hostingContext; OptionsServiceCollectionExtensions.PostConfigure<HostFilteringOptions>(services, new Action<HostFilteringOptions>(<>c__DisplayClass8_.<CreateDefaultBuilder>b__5)); ServiceCollectionServiceExtensions.AddSingleton<IOptionsChangeTokenSource<HostFilteringOptions>>(services, new ConfigurationChangeTokenSource<HostFilteringOptions>(<>c__DisplayClass8_.hostingContext.get_Configuration())); ServiceCollectionServiceExtensions.AddTransient<IStartupFilter, HostFilteringStartupFilter>(services); }
以下是Action<WebHostBuilderContext, ServiceProviderOptions> arg_BD_1;委托的逻辑
internal void <CreateDefaultBuilder>b__8_4(WebHostBuilderContext context, ServiceProviderOptions options) { options.set_ValidateScopes(HostingEnvironmentExtensions.IsDevelopment(context.get_HostingEnvironment())); }
以下是Action<WebHostBuilderContext, ILoggingBuilder> arg_70_1委托的逻辑
internal void <CreateDefaultBuilder>b__8_2(WebHostBuilderContext hostingContext, ILoggingBuilder logging) { LoggingBuilderExtensions.AddConfiguration(logging, hostingContext.get_Configuration().GetSection("Logging")); logging.AddConsole(); DebugLoggerFactoryExtensions.AddDebug(logging); }
由以上代码我们可以得出以下结论：
a.添加Kestrel和IIS的web服务器
b.添加Configuration配置系统
支持 appsettings.json 和多环境的 appsettings.{0}.json
支持CommandLine
c.添加日志系统（这里通过反编译WebHostBuilderExtensions扩展类，可以知道是调用了IServiceCollection的AddLogging扩展方法，然后我们转到 LoggingServiceCollectionExtensions扩展类，找到AddLogging扩展方法，然后将日志注入到IServiceCollection）
默认添加对Console和Debug的输出
3、UseStartup：通过反编译查看源代码
public static IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType) { string str = IntrospectionExtensions.GetTypeInfo(startupType).get_Assembly().GetName().get_Name(); return hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, str).ConfigureServices(delegate (IServiceCollection services) { if (IntrospectionExtensions.GetTypeInfo((Type) typeof(IStartup)).IsAssignableFrom(IntrospectionExtensions.GetTypeInfo(startupType))) { services.AddSingleton(typeof(IStartup), startupType); } else { services.AddSingleton(typeof(IStartup), delegate (IServiceProvider sp) { IHostingEnvironment requiredService = sp.GetRequiredService<IHostingEnvironment>(); return new ConventionBasedStartup(StartupLoader.LoadMethods(sp, startupType, requiredService.EnvironmentName)); }); } }); }
由上代码可得出结论：最终将Startup类添加到服务中。
4、Build：反编译查看源代码
public IWebHost Build() { AggregateException exception; IWebHost host2; if (this._webHostBuilt) { throw new InvalidOperationException(Microsoft.AspNetCore.Hosting.Resources.WebHostBuilder_SingleInstance); } this._webHostBuilt = true; IServiceCollection serviceCollection = this.BuildCommonServices(out exception); IServiceCollection services = serviceCollection.Clone(); IServiceProvider provider = <Build>g__GetProviderFromFactory|13_0(serviceCollection); if (!this._options.SuppressStatusMessages) { if (Environment.GetEnvironmentVariable("Hosting:Environment") != null) { Console.WriteLine("The environment variable 'Hosting:Environment' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'"); } if (Environment.GetEnvironmentVariable("ASPNET_ENV") != null) { Console.WriteLine("The environment variable 'ASPNET_ENV' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'"); } if (Environment.GetEnvironmentVariable("ASPNETCORE_SERVER.URLS") != null) { Console.WriteLine("The environment variable 'ASPNETCORE_SERVER.URLS' is obsolete and has been replaced with 'ASPNETCORE_URLS'"); } } ILogger<WebHost> requiredService = provider.GetRequiredService<ILogger<WebHost>>(); foreach (IGrouping<string, string> grouping in from g in Enumerable.GroupBy<string, string>(this._options.GetFinalHostingStartupAssemblies(), delegate (string a) { return a; }, (IEqualityComparer<string>) StringComparer.get_OrdinalIgnoreCase()) select g) { requiredService.LogWarning(string.Format("The assembly {0} was specified multiple times. Hosting startup assemblies should only be specified once.", grouping), Array.Empty<object>()); } this.AddApplicationServices(services, provider); WebHost host = new WebHost(services, provider, this._options, this._config, exception); try { host.Initialize(); host2 = host; } catch { host.Dispose(); throw; } return host2; }
由以上代码可以看出，该方法就是用来初始化操作的。
a.初始化了服务容器（BuildCommonServices这个方法中初始化容器时，注册了大把的服务）
b.初始化了WebHost(host.Initialize();)
c.我们反编译一个Initialize方法， this._startup = this._hostingServiceProvider.GetService<IStartup>(); 最终可以看到，是从服务中获取的。那么是时候
注入进去的呢，当然是UseStartup<T>这个方法注入的。
5、Run
这个方法就是用来运行Web应用程序并阻止调用线程，直到host停止。因为在运行之前要做很多的事情。（在启动之前做了无数的事情）
Startup类
public class Startup { public void ConfigureServices(IServiceCollection services) { } public void Configure(IApplicationBuilder app, IHostingEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.Run(async (context) => { await context.Response.WriteAsync("Hello World!"); }); } }
1、ConfigureServices：这个方法是运行是调用的，使用这个方法为程序的服务容器添加服务的
a.上面已经讲过，在Program类中的IWebHostBuilder中的Build()方法里面，程序初始化服务容器的时候就已经为我们添加了许多的服务，所以说，这个方法只是为了给开发
人员进行扩展。
2、Configure：这个方法是运行是调用的，这个方法是用来配置HTTP请求管道的
a.可以在管道中添加自定义或者Microsoft封装好的中间件
b.可以根据环境的不同，选择不同的中间件
通过编译追踪，最终调用的是IApplicationBuilder中的Use(Func<RequestDelegate, RequestDelegate> middleware);方法，而这个方法做的事情就是，将中间件添加到这个类型
的_components集合中。反编译代码如下：
private readonly IList<Func<RequestDelegate, RequestDelegate>> _components; public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware) { this._components.Add(middleware); return this; }
launchSettings.json启动文件
解决问题：快速的做环境切换，方便开发和调试
为了让你的程序在多环境一样保持稳定
wwwroot
用来存放静态资源。