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);
}
}