Question

I’m implementing a REST API using ASP.NET MVC, and a little stumbling block has come up in the form of the Expect: 100-continue request header for requests with a post body.

RFC 2616 states that:

Upon receiving a request which
includes an Expect request-header
field with the “100-continue” expectation, an origin server MUST
either respond with 100 (Continue) status and continue to read
from the input stream, or respond with a final status code. The
origin server MUST NOT wait for the request body before sending
the 100 (Continue) response. If it responds with a final status
code, it MAY close the transport connection or it MAY continue
to read and discard the rest of the request. It MUST NOT
perform the requested method if it returns a final status code.

This sounds to me like I need to make two responses to the request, i.e. it needs to immediately send a HTTP 100 Continue response, and then continue reading from the original request stream (i.e. HttpContext.Request.InputStream) without ending the request, and then finally sending the resultant status code (for the sake of argument, lets say it’s a 204 No Content result).

So, questions are:

1. Am I reading the specification right, that I need to make two responses to a request?
2. How can this be done in ASP.NET MVC?

w.r.t. (2) I have tried using the following code before proceeding to read the input stream…

HttpContext.Response.StatusCode = 100;
HttpContext.Response.Flush();
HttpContext.Response.Clear();

…but when I try to set the final 204 status code I get the error:

System.Web.HttpException: Server cannot set status after HTTP headers have been sent.

Answer 1

100-continue should be handled by IIS. Is there a reason why you want to do this explicitly?