asp.net mvc not getting value from ajax when converting data into json.stringify - ajax

Question

I am using a event handler that checks if the product id or name already exist. But my problem is when I am using JSON.stringify() my C# controller does not receive the data from the ajax call,.
// check if Product name already exist
$('#productName').bind('keyup blur', function () {
// check if input is empty
if ($(this).val().length > 0) {
var data = JSON.stringify({
value: $(this).val(),
fieldName: 'productName'
});
$.ajax({
type: "post",
url: '/Product/ValidateProductDetailsExist',
contenttype: "application/json; charset=utf-8",
datatype: "json",
data: data,
context: this,
success: function (result) {
if (result === true) {
// append error message
// check if error message already exist
if ($('#errorprodcutName').length === 0) {
var errormessage = '<div class="col-md-offset-2"><span id = "errorprodcutName" class="validation-error-message">Product name already exist</span></div >';
$('.form-group:nth-child(2)').append(errormessage);
}
$(this).focus();
//disables the save button
$('#btnSaveProduct').prop('disabled', true);
}
else {
// check if error message already exist
if ($('#errorprodcutName').length > 0) {
$('#errorprodcutName').remove();
}
//enables the save button
$('#btnSaveProduct').prop('disabled', false);
}
},
error: function () {
alert("unable to request from server");
}
});
}
});
When I use debugger to check the value, it is null. I don't see any errors that displays in the console as well. Can anyone please explain to me why it is not working.
public JsonResult ValidateProductDetailsExist(string value, string fieldName)
{
using (POSEntities3 db = new POSEntities3())
{
bool isExist = false;
switch (fieldName)
{
case "productId":
var dataItemProductId = db.Products.Where(product => product.product_id == value).SingleOrDefault();
isExist = (dataItemProductId != null);
break;
case "productName":
var dataItemProductName = db.Products.Where(product => product.name == value).SingleOrDefault();
isExist = (dataItemProductName != null);
break;
}
return Json(isExist, JsonRequestBehavior.AllowGet);
}
}