ASP.NET Core中间件计算Http请求时间示例详解

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);
  }
}

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