Why is Request.ContentType always empty on my requests?

Question

I have a base controller class that attempts to look at Request.ContentType to see if its a json request or a regular HTML request. The base controller then sets a simple enum on the base class and the appropriate controller returns the correct type. However, Request.ContentType is always an empty string. why is that?

my base controller:

namespace PAW.Controllers
{
    public class BaseController : Controller
    {
        public ResponseFormat ResponseFormat { get; private set; }
        public User CurrentUser { get; private set; }

        protected override void OnActionExecuting(ActionExecutingContext filterContext)
        {
            base.OnActionExecuting(filterContext);

            Culture.SetCulture();

            if (this.Request.ContentType.ToLower() == "application/json")
                this.ResponseFormat = ResponseFormat.Json;
            else
                this.ResponseFormat = ResponseFormat.Html;

            Response.Cache.SetCacheability(HttpCacheability.NoCache);

            //setup user object
            this.CurrentUser = WebUser.CurrentUser;
            ViewData["CurrentUser"] = WebUser.CurrentUser;
        }
    }

    public enum ResponseFormat
    {
        Html,
        Json,
        Xml
    }
}

my jquery:

$.ajax({
    type: "GET",
    contentType: "application/json; charset=utf-8",
    url: "/Vendor/Details/" + newid,
    data: "{}",
    dataType: "json",
    success: function(data) { ShowVendor(data); },
    error: function(req, status, err) { ShowError("Sorry, an error occured (Vendor callback failed).  Please try agian later."); }
});

Answer 1

The "content-type" header doesn't do anything when your method is "GET".

Answer 2

Try using

 $.ajax({
            type: "GET",
            contentType: "application/json; charset=utf-8",
            url: "/Vendor/Details/" + newid,
            beforeSend: function(xhr) {
                xhr.setRequestHeader("Content-type",
                     "application/json; charset=utf-8");
            },
            data: "{}",
            dataType: "json",
            success: function(data) { alert(data); },
            error: function(req, status, err) { ShowError("Sorry, an error occured (Vendor callback failed).  Please try agian later."); }
        });

a la - http://encosia.com/2008/06/05/3-mistakes-to-avoid-when-using-jquery-with-aspnet-ajax/

Answer 3

It looks like you're trying to use the ContentType header to determine which type of response to return. That's not what it's for. You should be using the Accepts header instead, which tells the server which content types you accept.