本文主要是对.NET Core开发日志——Middleware的补遗，但是会从看起来平平无奇的RequestDelegate开始叙述，所以以其作为标题，也是合情合理。
RequestDelegate是一种委托类型，其全貌为public delegate Task RequestDelegate(HttpContext context)，MSDN上对它的解释，"A function that can process an HTTP request."——处理HTTP请求的函数。唯一参数，是最熟悉不过的HttpContext，返回值则是表示请求处理完成的异步操作类型。
可以将其理解为ASP.NET Core中对一切HTTP请求处理的抽象(委托类型本身可视为函数模板，其实现具有统一的参数列表及返回值类型)，没有它整个框架就失去了对HTTP请求的处理能力。
并且它也是构成Middleware的基石。或者更准确地说参数与返回值都是其的Func<RequestDelegate, RequestDelegate>委托类型正是维持Middleware运转的核心齿轮。
组装齿轮的地方位于ApplicationBuilder类之内，其中包含着所有齿轮的集合。
private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();以及添加齿轮的方法：
public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware){ ???_components.Add(middleware); ???return this;}在Startup类的Configure方法里调用以上ApplicationBuilder的Use方法，就可以完成一个最简单的Middleware。
public void Configure(IApplicationBuilder app){ ???app.Use(_ => ???{ ???????return context => ???????{ ???????????return context.Response.WriteAsync("Hello, World!"); ???????}; ???????????});}齿轮要想变成Middleware，在完成添加后，还需要经过组装。
public RequestDelegate Build(){ ???RequestDelegate app = context => ???{ ???????context.Response.StatusCode = 404; ???????return Task.CompletedTask; ???}; ???foreach (var component in _components.Reverse()) ???{ ???????app = component(app); ???} ???return app;}Build方法里先定义了最底层的零件——app，context => { context.Response.StatusCode = 404; return Task.CompletedTask; }，这段代码意味着，如果没有添加任何Middleware的话，ASP.NET Core站点启动后，会直接出现404的错误。
接下的一段，遍历倒序排列的齿轮，开始正式组装。
在上述例子里，只使用了一个齿轮：
_ =>{ ???return context => ???{ ???????return context.Response.WriteAsync("Hello, World!"); ???}; ???}那么第一次也是最后一次循环后，执行component(app)操作，app被重新赋值为：
context => context.Response.WriteAsync("Hello, World!");组装的结果便是app的值。
这个组装过程在WebHost进行BuildApplication时开始操作。从此方法的返回值类型可以看出，虽然明义上是创建Application，其实生成的是RequestDelegate。
private RequestDelegate BuildApplication(){ ???try ???{ ???????... ???????var builderFactory = _applicationServices.GetRequiredService<IApplicationBuilderFactory>(); ???????var builder = builderFactory.CreateBuilder(Server.Features); ???????... ???????Action<IApplicationBuilder> configure = _startup.Configure; ???????... ???????configure(builder); ???????return builder.Build(); ???} ???...} ???而这个RequestDelegate最终会在HostingApplication类的ProcessRequestAsync方法里被调用。
public virtual async Task StartAsync(CancellationToken cancellationToken = default){ ???... ???var application = BuildApplication(); ???... ???var hostingApp = new HostingApplication(application, _logger, diagnosticSource, httpContextFactory); ???...} ???public HostingApplication( ???RequestDelegate application, ???ILogger logger, ???DiagnosticListener diagnosticSource, ???IHttpContextFactory httpContextFactory){ ???_application = application; ???_diagnostics = new HostingApplicationDiagnostics(logger, diagnosticSource); ???_httpContextFactory = httpContextFactory;}public Task ProcessRequestAsync(Context context){ ???return _application(context.HttpContext);}上例中的执行结果即是显示Hello, World!字符。
404的错误不再出现，意味着这种Middleware只会完成自己对HTTP请求的处理，并不会将请求传至下一层的Middleware。
要想达成不断传递请求的目的，需要使用另一种Use扩展方法。
public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware){ ???return app.Use(next => ???{ ???????return context => ???????{ ???????????Func<Task> simpleNext = () => next(context); ???????????return middleware(context, simpleNext); ???????}; ???});}在实际代码中可以这么写：
public void Configure(IApplicationBuilder app){ ???app.Use(async (context, next) => ???{ ???????await context.Response.WriteAsync("I am a Middleware!\n"); ???????await next.Invoke(); ???}); ???app.Use(_ => ???{ ???????return context => ???????{ ???????????return context.Response.WriteAsync("Hello, World!"); ???????}; ???});}现在多了个Middleware，继续上面的组装过程。app的值最终被赋值为：
async context =>{ ???Func<Task> simpleNext = () => context.Response.WriteAsync("Hello, World!"); ????await context.Response.WriteAsync("I am a Middleware!\n"); ???await simpleNext.Invoke();};显示结果为：
I am a Middleware!Hello, World!下面的流程图中可以清楚地说明这个过程。
如果把await next.Invoke()注释掉的话，
public void Configure(IApplicationBuilder app){ ???app.Use(async (context, next) => ???{ ???????await context.Response.WriteAsync("I am a Middleware!\n"); ???????//await next.Invoke(); ???}); ???app.Use(_ => ???{ ???????return context => ???????{ ???????????return context.Response.WriteAsync("Hello, World!"); ???????}; ???});}上例中第一个Middleware处理完后，不会继续交给第二个Middleware处理。注意以下simpleNext的方法只被定义而没有被调用。
async context =>{ ???Func<Task> simpleNext = () => context.Response.WriteAsync("Hello, World!"); ????await context.Response.WriteAsync("I am a Middleware!\n");};这种情况被称为短路(short-circuiting)。
做短路处理的Middleware一般会放在所有Middleware的最后，以作为整个pipeline的终点。
并且更常见的方式是用Run扩展方法。
public static void Run(this IApplicationBuilder app, RequestDelegate handler){ ???... ???app.Use(_ => handler);}所以可以把上面例子的代码改成下面的形式：
public void Configure(IApplicationBuilder app){ ???app.Use(async (context, next) => ???{ ???????await context.Response.WriteAsync("I am a Middleware!\n"); ???????await next.Invoke(); ???}); ???app.Run(async context => ???{ ???????await context.Response.WriteAsync("Hello, World!"); ???});}除了短路之外，Middleware处理时还可以有分支的情况。
public void Configure(IApplicationBuilder app){ ???app.Map("/branch1", ab => { ???????ab.Run(async context => ???????{ ???????????await context.Response.WriteAsync("Map branch 1"); ???????}); ???}); ???app.Map("/branch2", ab => { ???????ab.Run(async context => ???????{ ???????????await context.Response.WriteAsync("Map branch 2"); ???????}); ???}); ???app.Use(async (context, next) => ???{ ???????await context.Response.WriteAsync("I am a Middleware!\n"); ???????await next.Invoke(); ???}); ???app.Run(async context => ???{ ???????await context.Response.WriteAsync("Hello, World!"); ???});}URL地址后面跟着branch1时：
URL地址后面跟着branch2时：
其它情况下：
Map扩展方法的代码实现：
public static IApplicationBuilder Map(this IApplicationBuilder app, PathString pathMatch, Action<IApplicationBuilder> configuration){ ???... ???// create branch ???var branchBuilder = app.New(); ???configuration(branchBuilder); ???var branch = branchBuilder.Build(); ???var options = new MapOptions ???{ ???????Branch = branch, ???????PathMatch = pathMatch, ???}; ???return app.Use(next => new MapMiddleware(next, options).Invoke);}创建分支的办法就是重新实例化一个ApplicationBuilder。
public IApplicationBuilder New(){ ???return new ApplicationBuilder(this);}对分支的处理则是封装在MapMiddleware类之中。
public async Task Invoke(HttpContext context){ ???... ???PathString matchedPath; ???PathString remainingPath; ???if (context.Request.Path.StartsWithSegments(_options.PathMatch, out matchedPath, out remainingPath)) ???{ ???????// Update the path ???????var path = context.Request.Path; ???????var pathBase = context.Request.PathBase; ???????context.Request.PathBase = pathBase.Add(matchedPath); ???????context.Request.Path = remainingPath; ???????try ???????{ ???????????await _options.Branch(context); ???????} ???????finally ???????{ ???????????context.Request.PathBase = pathBase; ???????????context.Request.Path = path; ???????} ???} ???else ???{ ???????await _next(context); ???}}说到MapMiddleware，不得不提及各种以Use开头的扩展方法，比如UseStaticFiles，UseMvc，UsePathBase等等。
这些方法内部都会调用UseMiddleware方法以使用各类定制的Middleware类。如下面UsePathBase的代码：
public static IApplicationBuilder UsePathBase(this IApplicationBuilder app, PathString pathBase){ ???... ???// Strip trailing slashes ???pathBase = pathBase.Value?.TrimEnd(‘/‘); ???if (!pathBase.HasValue) ???{ ???????return app; ???} ???return app.UseMiddleware<UsePathBaseMiddleware>(pathBase);}而从UseMiddleware方法中可以获知，Middleware类需满足两者条件之一才能被有效使用。其一是实现IMiddleware，其二，必须有Invoke或者InvokeAsync方法，且方法至少要有一个HttpContext类型参数(它还只能是放第一个)，同时返回值需要是Task类型。
internal const string InvokeMethodName = "Invoke";internal const string InvokeAsyncMethodName = "InvokeAsync";public static IApplicationBuilder UseMiddleware(this IApplicationBuilder app, Type middleware, params object[] args){ ???if (typeof(IMiddleware).GetTypeInfo().IsAssignableFrom(middleware.GetTypeInfo())) ???{ ???????... ???????return UseMiddlewareInterface(app, middleware); ???} ???var applicationServices = app.ApplicationServices; ???return app.Use(next => ???{ ???????var methods = middleware.GetMethods(BindingFlags.Instance | BindingFlags.Public); ???????var invokeMethods = methods.Where(m => ???????????string.Equals(m.Name, InvokeMethodName, StringComparison.Ordinal) ???????????|| string.Equals(m.Name, InvokeAsyncMethodName, StringComparison.Ordinal) ???????????).ToArray(); ???????... ???????var ctorArgs = new object[args.Length + 1]; ???????ctorArgs[0] = next; ???????Array.Copy(args, 0, ctorArgs, 1, args.Length); ???????var instance = ActivatorUtilities.CreateInstance(app.ApplicationServices, middleware, ctorArgs); ???????if (parameters.Length == 1) ???????{ ???????????return (RequestDelegate)methodinfo.CreateDelegate(typeof(RequestDelegate), instance); ???????} ???????var factory = Compile<object>(methodinfo, parameters); ???????return context => ???????{ ???????????var serviceProvider = context.RequestServices ?? applicationServices; ???????????... ???????????return factory(instance, context, serviceProvider); ???????}; ???});}对ASP.NET Core中Middleware的介绍到此终于可以告一段落，希望这两篇文章能够为读者提供些许助力。
.NET Core开发日志——RequestDelegate
原文地址：https://www.cnblogs.com/kenwoo/p/9404671.html