How to specify ContentType for Web API controller method

There's a Request object, and getting the request content type is easy. But how do you specify a content type for the response? My controller looks like this (other actions excised for brevity):

public class AuditController : ApiController
{   
  // GET api/Audit/CSV
  [HttpGet, ActionName("CSV")]
  public string Csv(Guid sessionId, DateTime a, DateTime b, string predicate)
  {
    var result = new StringBuilder();
    //build a string
    return result.ToString();
  }
}

This works fine except that it has the wrong content type. I'd like to do this

Response.ContentType = "text/csv";

A little research reveals that we can type the Action to return an HttpResponseMessage. So the end of my method would look like this:

  var response = new HttpResponseMessage() ;
  response.Headers.Add("ContentType","text/csv");
  response.Content = //not sure how to set this
  return response;

The documentation on HttpContent is rather sparse, can anyone advise me on how to get the contents of my StringBuilder into an HttpContent object?

You'll have to change the return type of the method to HttpResponseMessage, then use Request.CreateResponse:

// GET api/Audit/CSV
[HttpGet, ActionName("CSV")]
public HttpResponseMessage Csv(Guid sessionId, DateTime a, DateTime b, string predicate)
{
    var result = new StringBuilder();

    //build a string

    var res = Request.CreateResponse(HttpStatusCode.OK);
    res.Content = new StringContent(result.ToString(), Encoding.UTF8, "text/csv");

    return res;
}