Understanding SendAsync method ASP.net web API Message Handlers -

i trying implement custom message handler, below code.

public class myhandler : delegatinghandler {     protected override async task<httpresponsemessage> sendasync(httprequestmessage request, system.threading.cancellationtoken cancellationtoken)     {         /* code executes while receiving request - start */         bool isbadrequest = false;          if (isbadrequest)         {             return request.createresponse(system.net.httpstatuscode.notfound,"test");         }        /* code executes while receiving request - end */         /* code executes while sending response - start */                     var response = await base.sendasync(request , cancellationtoken);         response.headers.add("new header", "new header value");         return response;        /* code executes while sending response - end */     }  } 

i aware of tasks , async/await in c#. unable understand how same method gets executed both while receiving request , sending response.

this right asp.net website , explains how sendasync works.

the method takes httprequestmessage input , asynchronously returns httpresponsemessage. typical implementation following:

1. process request message.
2. call base.sendasync send request inner handler.
3. the inner handler returns response message. (this step asynchronous.)
4. process response , return caller.

here trivial example:

public class messagehandler1 : delegatinghandler {     protected async override task<httpresponsemessage> sendasync(         httprequestmessage request, cancellationtoken cancellationtoken)     {         debug.writeline("process request");         // call inner handler.         var response = await base.sendasync(request, cancellationtoken);         debug.writeline("process response");         return response;     } } 

furthermore, here great article how log request , response messages using message handler.