之前ASP.NET中使用的HTTP Modules及HTTP Handlers，在ASP.NET Core中已不复存在，取而代之的是Middleware。Middleware除了简化了HTTP Modules/Handlers的使用方式，还带入了Pipeline的概念。
本篇将介绍ASP.NET Core的Middleware概念及用法。Middleware 概念
ASP.NET Core在Middleware的官方说明中，使用了Pipeline这个名词，意指Middleware像水管一样可以串联在一起，所有的Request及Response都会层层经过这些水管。用图例可以很容易理解，如下图：
App.Use
Middleware的注册方式是在Startup.cs的Configure对IApplicationBuilder使用Use方法注册。
- UseMvc()：MVC的Middleware
- UseRewriter()：URL rewriting的Middleware
一个简单的Middleware 范例。如下：
Startup.cs
using System;using System.Collections.Generic;using System.Linq;using System.Threading.Tasks;using Microsoft.AspNetCore.Builder;using Microsoft.AspNetCore.Hosting;using Microsoft.AspNetCore.Http;using Microsoft.Extensions.DependencyInjection;namespace MyWebsite{ public class Startup { // This method gets called by the runtime. Use this method to add services to the container. // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940 public void ConfigureServices(IServiceCollection services) { } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IHostingEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.Use(async (context, next) => { await context.Response.WriteAsync("First Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("First Middleware out. \r\n"); }); app.Use(async (context, next) => { await context.Response.WriteAsync("Second Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("Second Middleware out. \r\n"); }); app.Use(async (context, next) => { await context.Response.WriteAsync("Third Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("Third Middleware out. \r\n"); }); app.Run(async (context) => { await context.Response.WriteAsync("Hello World! \r\n"); }); } }} 用浏览器打开网站任意连结，输出结果：
First Middleware in. Second Middleware in. Third Middleware in. Hello World! Third Middleware out. Second Middleware out. First Middleware out.
在Pipeline的概念中，注册顺序是很重要的事情。请求经过的顺序一定是先进后出。
Request 流程如下图：
Middleware 也可以作为拦截使用，如下：
Startup.cs
using System;using System.Collections.Generic;using System.Linq;using System.Threading.Tasks;using Microsoft.AspNetCore.Builder;using Microsoft.AspNetCore.Hosting;using Microsoft.AspNetCore.Http;using Microsoft.Extensions.DependencyInjection;namespace MyWebsite{ public class Startup { // This method gets called by the runtime. Use this method to add services to the container. // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940 public void ConfigureServices(IServiceCollection services) { } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IHostingEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.Use(async (context, next) => { await context.Response.WriteAsync("First Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("First Middleware out. \r\n"); }); app.Use(async (context, next) => { await context.Response.WriteAsync("Second Middleware in. \r\n"); // 水管阻塞，封包不往后送 var condition = false; if (condition) { await next.Invoke(); } await context.Response.WriteAsync("Second Middleware out. \r\n"); }); app.Use(async (context, next) => { await context.Response.WriteAsync("Third Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("Third Middleware out. \r\n"); }); app.Run(async (context) => { await context.Response.WriteAsync("Hello World! \r\n"); }); } }} 输出结果：
First Middleware in. Second Middleware in. Second Middleware out. First Middleware out.
在Second Middleware 中，因为没有达成条件，所以封包也就不在往后面的水管传送。流程如图：
App.Run
Run是Middleware的最后一个行为，以上面图例来说，就是最末端的Action。
Use能串联其他Middleware，但Run还是能完整的使用Request及Response。 App.Map
Map是能用来处理一些简单路由的Middleware，可依照不同的URL指向不同的Run及注册不同的Use。新增一个路由如下：
Startup.cs
using System;using System.Collections.Generic;using System.Linq;using System.Threading.Tasks;using Microsoft.AspNetCore.Builder;using Microsoft.AspNetCore.Hosting;using Microsoft.AspNetCore.Http;using Microsoft.Extensions.DependencyInjection;namespace MyWebsite{ public class Startup { // This method gets called by the runtime. Use this method to add services to the container. // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940 public void ConfigureServices(IServiceCollection services) { } // This method gets called by the runtime. Use this method to configure the HTTP request pipeline. public void Configure(IApplicationBuilder app, IHostingEnvironment env) { if (env.IsDevelopment()) { app.UseDeveloperExceptionPage(); } app.Use(async (context, next) => { await context.Response.WriteAsync("First Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("First Middleware out. \r\n"); }); // app.Use(async (context, next) => // { // await context.Response.WriteAsync("Second Middleware in. \r\n"); // // 水管阻塞，封包不往后送 // var condition = false; // if (condition) // { // await next.Invoke(); // } // await context.Response.WriteAsync("Second Middleware out. \r\n"); // }); app.Map("/second", mapApp => { mapApp.Use(async (context, next) => { await context.Response.WriteAsync("Second Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("Second Middleware out. \r\n"); }); mapApp.Run(async context => { await context.Response.WriteAsync("Second. \r\n"); }); }); app.Use(async (context, next) => { await context.Response.WriteAsync("Third Middleware in. \r\n"); await next.Invoke(); await context.Response.WriteAsync("Third Middleware out. \r\n"); }); app.Run(async (context) => { await context.Response.WriteAsync("Hello World! \r\n"); }); } }} 开启网站任意连结，会显示：
First Middleware in. Third Middleware in. Hello World! Third Middleware out. First Middleware out.
开启网站http://localhost:5000/second，则会显示：
First Middleware in. Second Middleware in. Second. Second Middleware out. First Middleware out.
创建Middleware 类
如果Middleware全部都写在Startup.cs，代码将很难维护，所以应该把自定义的Middleware逻辑独立出来。
建立Middleware类不需要额外继承其它类或接口，一般的类即可，例子如下：FirstMiddleware.cs
using System.Threading.Tasks;using Microsoft.AspNetCore.Http;namespace MyWebsite{ public class FirstMiddleware { private readonly RequestDelegate _next; public FirstMiddleware(RequestDelegate next) { _next = next; } public async Task Invoke(HttpContext context) { await context.Response.WriteAsync($"{nameof(FirstMiddleware)} in. \r\n"); await _next(context); await context.Response.WriteAsync($"{nameof(FirstMiddleware)} out. \r\n"); } }} 全局注册
在Startup.Configure注册Middleware就可以套用到所有的Request。如下：
Startup.cs
// ...public class Startup{ // ... public void Configure(IApplicationBuilder app) { app.UseMiddleware (); // ... }} 局部注册
Middleware 也可以只套用在特定的Controller 或Action。注册方式如下：
Controllers\HomeController.cs
// ..[MiddlewareFilter(typeof(FirstMiddleware))]public class HomeController : Controller{ // ... [MiddlewareFilter(typeof(SecondMiddleware))] public IActionResult Index() { // ... }} Extensions
大部分扩展的Middleware都会用一个静态方法包装，如：UseMvc()、UseRewriter()等。
Extensions\CustomMiddlewareExtensions.cs
using Microsoft.AspNetCore.Builder;namespace MyWebsite{ public static class CustomMiddlewareExtensions { public static IApplicationBuilder UseFirstMiddleware(this IApplicationBuilder builder) { return builder.UseMiddleware (); } }} 注册Extension Middleware 的方式如下：
Startup.cs
// ...public class Startup{ // ... public void Configure(IApplicationBuilder app) { app.UseFirstMiddleware(); // ... }} 参考
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