Unable to return List from AJAX call MVC ? Following one example mentioned

Question

I am trying to return a list from MVC controller. Value is returned easily when I am returning a hard code value. But when I am returning a list, it gives me an error. Here is my code ,

Ajax call,

function MyFunction() {
    alert($('#DDlSurvey').val());
    $.ajax({
        url: "@Url.Action("Index", "ConductSurvey")",
        data: { prefix: $('#DDlSurvey').val() },
        type: "POST",
        dataType: "json",
        success: function (data) {
             // loadData(data);
            alert("Success");
           // alert(data)
        },
        error: function (data){
            alert("Failed! Please try again.");
        }
    });
    //$('#YourLabelId').val('ReplaceWithThisValue');
}

and my method is,

[HttpPost]
public JsonResult Index(int prefix)
{

    List<SelectList> Questions = new List<SelectList>();
    List<Question> QuestionList = new List<Question>();
    List<string> ll = new List<string>();
    Question nn = new Question();
    SurveyAppEntities ObjectSur = new SurveyAppEntities();

    QuestionList = (from q in ObjectSur.Questions
                    join b in ObjectSur.SurveyQuestions on q.ID equals b.QuestionID
                    where b.SurveyID.Equals(prefix)
                    select q).ToList();
   //return Json("OK");
  return new JsonResult  {Data=QuestionList, JsonRequestBehavior=JsonRequestBehavior.AllowGet};
}

When I return OK i get it but when i try to return QuestionList It is not returning data and display Failed

I am also using one link for help in which every thing is as it is link is,

http://www.dotnetawesome.com/2014/05/how-to-retrieve-database-data-show-using-jquery-mvc-asp.html

Hopes for your suggestion

Answer 1

A circular reference was detected indicates that EF generates proxy objects when generating query results which cannot be serialized using JsonResult. Assumed that SurveyAppEntities is your DbContext instance, then you need to do one of 2 options below:

1) Disable proxy creation

You need to set ProxyCreationEnabled property to false which prevent EF creating proxy objects which cannot be serialized:

[HttpPost]
public JsonResult Index(int prefix)
{

    List<SelectList> Questions = new List<SelectList>();
    List<Question> QuestionList = new List<Question>();
    List<string> ll = new List<string>();
    Question nn = new Question();
    SurveyAppEntities ObjectSur = new SurveyAppEntities();

    // this line is mandatory
    ObjectSur.Configuration.ProxyCreationEnabled = false;

    QuestionList = (from q in ObjectSur.Questions
                    join b in ObjectSur.SurveyQuestions on q.ID equals b.QuestionID
                    where b.SurveyID.Equals(prefix)
                    select q).ToList();

    return Json(QuestionList, JsonRequestBehavior.AllowGet);
}

2) Disable query result tracking

EF implements query result tracking with proxy objects which may be disabled by adding AsNoTracking() to each table entities:

[HttpPost]
public JsonResult Index(int prefix)
{

    List<SelectList> Questions = new List<SelectList>();
    List<Question> QuestionList = new List<Question>();
    List<string> ll = new List<string>();
    Question nn = new Question();
    SurveyAppEntities ObjectSur = new SurveyAppEntities();

    QuestionList = (from q in ObjectSur.Questions
                    join b in ObjectSur.SurveyQuestions on q.ID equals b.QuestionID
                    where b.SurveyID.Equals(prefix)
                    select q).AsNoTracking().ToList(); // prevents result tracking

    return Json(QuestionList, JsonRequestBehavior.AllowGet);
}

Note:

You may also try to return only required properties instead of entire ObjectSur.Questions object.

Related issue:

Circular reference detected exception while serializing object to JSON

Answer 2

Simply return list like below

return Json(QuestionList, JsonRequestBehavior.AllowGet);