Question

MVC 2

I’m trying to post to an MVC action using jQuery, but I’m getting an exception stating that id is null. Do I need some sort of attribute on the controller action to accept json packets like this? Something else I’m missing?

[HttpPost]
public ActionResult SearchCategory(int id, string SearchTerm, int PageNumber, int SiteIdFilter)
{
    return Json(AssociationsDao.SearchCategory(id, SearchTerm, PageNumber, SiteIdFilter));
}

---

post('<%= Url.Action("SearchCategory") %>', 
    JSON.stringify({id: 12, SearchTerm: '', PageNumber: 1, SiteIdFilter: 1}), 
    function(d) { alert(d); });

function post(targetURL, dataInput, success) {
    $.ajax({
        url: targetURL,
        type: "POST",
        contentType: "application/json; charset=utf-8",
        data: dataInput,
        dataType: "json",
        success: success,
        async: true
    });
}

---

From the Chrome developer tools, this is the exception:

The parameters dictionary contains a null entry for parameter ‘id’ of
non-nullable type ‘System.Int32’ for method
‘System.Web.Mvc.ActionResult SearchCategory(Int32, System.String,
Int32, Int32)’ in
‘Current.Web.BackOffice.WebUI.Controllers.SiteProductAssociationsController’.
An optional parameter must be a reference type, a nullable type, or be
declared as an optional parameter. Parameter name: parameters

---

EDIT

Here’s a screenshot of the post data from chrome; I can’t see anything wrong with it:

Answer 1

Modify the post function to this:

function post(targetURL, dataInput, success) {
    $.ajax({
        url: targetURL,
        type: "POST",
        data: dataInput,
        success: success,
        async: true        
    });
}

MVC is smarter than asmx web services. Instead of insisting on a json string, you can, in fact need to, just pass a plain JavaScript object. See this answer for more information.

When you give jQuery’s $.ajax() function a data variable in the form
of a javascript object, it actually translates it to a series of
key/value pairs and sends it to the server in that manner. When you
send the stringified json object, it’s not in that form and the
standard model binder in mvc/mvc2 won’t cut it.

---

Also note that the async parameter is superfluous since true is the default value, but that won’t hurt anything.