Question

I’ve asked this question over on programmers that’s linked to this one. I’m trying to find a suitable header, that is unlikely to be stripped, that I can use to send back a unique Request ID with every response, even if it does not send a body.

One of the headers I considered was the Pragma header, as looking at the spec it appears to be intended not only for the additional no-cache HTTP 1.0 backwards-compatibility value, but also for application-specific values, so I should be able to use it. It should be possible, for example, to send something like no-cache; requestid=id.

So in a DelegatingHandler I tried writing to it with my ID:

//HttpResponseMessage Response; 
Response.Headers.Add("pragma", "some_value"); 

But it arrives at the client with no-cache; always. I think WebAPI automatically sends caching headers consistent with caching being switched off, which includes the Pragma one.

So, how do I make sure my value is maintained and not overwritten?

Answer 1

I’ve cracked it, the answer is to make sure you also set the CacheControl header on the HttpResponseMessage, which then bypasses some slightly fishy logic in System.Web.Http.WebHost.HttpControllerHandler (I’ve opened a discussion on CodePlex about this; I think the logic needs to be changed).

So instead of

//HttpResponseMessage Response;
Response.Headers.Add("pragma", "some_value");

You have to do:

Response.Headers.CacheControl = 
  new System.Net.Http.Headers.CacheControlHeaderValue() 
  { 
    NoCache = true 
  };
Response.Headers.Add("pragma", "some_value");

(I’ve used NoCache since the current API default is to switch caching off for all responses).