ASP.NET Core通过RequestDelegate这个委托类型来定义中间件
public delegate Task RequestDelegate(HttpContext context);可将一个单独的请求委托并行指定为匿名方法（称为并行中间件），或在类中对其进行定义。可通过Use，或在Middleware类中配置要传递给委托执行的方法(参数类型HttpContext，返回值类型Task)。
public static IApplicationBuilder Use(this IApplicationBuilder app, Func<HttpContext, Func<Task>, Task> middleware); public static IApplicationBuilder UseMiddleware<TMiddleware>(this IApplicationBuilder app, params object[] args);通过定义一个中间件类 来计算http请求的时间，例：
public class ResponseTimeMiddleware { // Name of the Response Header, Custom Headers starts with "X-" private const string RESPONSE_HEADER_RESPONSE_TIME = "X-Response-Time-ms"; // Handle to the next Middleware in the pipeline private readonly RequestDelegate _next; public ResponseTimeMiddleware(RequestDelegate next) { _next = next; } public Task InvokeAsync(HttpContext context) { // Start the Timer using Stopwatch var watch = new Stopwatch(); watch.Start(); context.Response.OnStarting(() => { // Stop the timer information and calculate the time watch.Stop(); var responseTimeForCompleteRequest = watch.ElapsedMilliseconds; // Add the Response time information in the Response headers. context.Response.Headers[RESPONSE_HEADER_RESPONSE_TIME] = responseTimeForCompleteRequest.ToString(); return Task.CompletedTask; }); // Call the next delegate/middleware in the pipeline return this._next(context); } }