熟悉ASP.NET架构的开发者一定对于HTTP Modules与HTTP Handlers不陌生。两者的作用主要是对网络请求执行特定的处理工作。而在.NET Core中，它们都被Middleware(中件间)取代了。
之前的Http Modules和HTTP Handlers是如下图般处理请求的：
现在变成了这样：
一言概括之，Middleware完成了HTTP Modules与HTTP Handlers的原有工作，但又不是简单的化二为一的减法作用。
Middleware减去的其实是与原来ASP.NET中重要的基础——应用程序生命周期事件(application life cycle event)的绑定。
HTTP Modules在初始化时就需要针对HttpApplication的事件作绑定处理，这样当HttpApplication的各项事件被触发时，已绑定的相应处理程序才会按照预期的那样被执行。
public class HelloWorldModule : IHttpModule{ ???public HelloWorldModule() ???{ ???} ???public String ModuleName ???{ ???????get { return "HelloWorldModule"; } ???} ???// In the Init function, register for HttpApplication ????// events by adding your handlers. ???public void Init(HttpApplication application) ???{ ???????application.BeginRequest += ????????????(new EventHandler(this.Application_BeginRequest)); ???????application.EndRequest += ????????????(new EventHandler(this.Application_EndRequest)); ???} ???private void Application_BeginRequest(Object source, ?????????EventArgs e) ???{ ???// Create HttpApplication and HttpContext objects to access ???// request and response properties. ???????HttpApplication application = (HttpApplication)source; ???????HttpContext context = application.Context; ???????context.Response.Write("<h1><font color=red> ???????????HelloWorldModule: Beginning of Request ???????????</font></h1><hr>"); ???} ???private void Application_EndRequest(Object source, EventArgs e) ???{ ???????HttpApplication application = (HttpApplication)source; ???????HttpContext context = application.Context; ???????context.Response.Write("<hr><h1><font color=red> ???????????HelloWorldModule: End of Request</font></h1>"); ???} ???public void Dispose() ???{ ???}}然后你还需要在web.config配置文件注册这个HTTP Module。
<configuration> ???<system.web> ???????<httpModules> ??????????<add name="HelloWorldModule" type="HelloWorldModule"/> ???????</httpModules> ???</system.web></configuration>如果是用Middleware的话，事情就变得很简单了。抛弃IHttpModule接口及HttpModule实现类，不用再关心HttpApplication的任何事件，还有烦人的web.config配置。直接在代码中以最简洁的方式完成工作。
???public void Configure(IApplicationBuilder app, IHostingEnvironment env){ ???app.Use(async(context, next) =>{ ???????await context.Response.WriteAsync("Beginning of Request\n"); ???????await next.Invoke(); ???????await context.Response.WriteAsync("End of Request\n"); ???}); ???app.Run(async (context) => ???{ ???????await context.Response.WriteAsync("Hello World!\n"); ???});}相似的，对于HTTP Handlers，虽然不用取消对HttpApplication事件的依赖，但以两者的代码实现方式作比较，Middleware亳无疑问胜出一筹。
public class HelloWorldHandler : IHttpHandler{ ???public HelloWorldHandler() ???{ ???} ???public void ProcessRequest(HttpContext context) ???{ ???????HttpRequest Request = context.Request; ???????HttpResponse Response = context.Response; ???????// This handler is called whenever a file ending ????????// in .sample is requested. A file with that extension ???????// does not need to exist. ???????Response.Write("<html>"); ???????Response.Write("<body>"); ???????Response.Write("<h1>Hello from a synchronous custom HTTP handler.</h1>"); ???????Response.Write("</body>"); ???????Response.Write("</html>"); ???} ???public bool IsReusable ???{ ???????// To enable pooling, return true here. ???????// This keeps the handler in memory. ???????get { return false; } ???}}仍需要在web.config文件中注册HTTP handler。
<configuration> ???<system.web> ???????<httpHandlers> ???????????<add verb="*" path="*.sample" ??????????????????type="HelloWorldHandler"/> ???????</httpHandlers> ???</system.web></configuration>换作Middleware的写法：
private static void HandleSample(IApplicationBuilder app){ ???app.Run(async context => ???{ ???????await context.Response.WriteAsync("Hello Sample"); ???});}// This method gets called by the runtime. Use this method to configure the HTTP request pipeline.public void Configure(IApplicationBuilder app, IHostingEnvironment env){ ???app.MapWhen(context => context.Request.Path.Value.EndsWith("sample"), HandleSample);}总结下使用Middleware的优点：
- 没有对HttpApplication的依赖
- 没有对IHttpModule与IHttpHandler接口的依赖
- 无需在web.config文件中添加各种配置
- 代码简洁
最后需要补充Middleware与HTTP Modules的一点差异。各Middleware中处理请求与响应的顺序是刚好相反的，越早处理请求的Middleware越晚处理响应。而HTTP Modules中处理请求与响应的顺序则保持一致，因为每个HTTP Module请求与响应事件的绑定都是在同一阶段完成的。
.NET Core开发日志——Middleware
原文地址：https://www.cnblogs.com/kenwoo/p/9275922.html