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);
 }
}

总结