HttpException in ASP.NET MVC with EntityFramework and JSON

Question

I want to make a select on the database. But I get following error (only the first rows of the error):

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below

[HttpException (0x80004005): Server cannot set content type after HTTP headers have been sent.]
   System.Web.HttpResponse.set_ContentType(String value) +9752569
   System.Web.HttpResponseWrapper.set_ContentType(String value) +14
   System.Web.Mvc.JsonResult.ExecuteResult(ControllerContext context) +177
   System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult) +27

A part of my DBcontroller:

string sql = @"exec [results].[GetNewResults] '" + Sources + @"', '" + Searchstring + @"', '" + username + @"', '" + sessionid + @"'";
        var result = dbcontext_hke.Database.SqlQuery<UnifiedResultset>(sql);

if (results == null)
{
    Debug.WriteLine("results is null");
    return Json(null);
}

Debug.WriteLine("results isn't null");
Debug.WriteLine(results);
return Json(results, JsonRequestBehavior.AllowGet);

A part of my js:

var url = $('#initialrequest').val();
$.ajax({
    url: url,
    data: {sources, searchstring},
    type: 'POST',
})
.success(function (result) {
for (var i = 0; i <= (9); i++) {
    try {
        console.log("lenght of result is :" + result.length);
        add_error(result);
        console.log("result is : " + result);
        add_result(result[i].header, result[i].source, result[i].subsource, result[i].description);
    } catch (err) {
        add_error();
    }
 })
.error(function (xhr, status) {
    $('#error').append(xhr.responseText);
    console.log(xhr.responseText);
});

Basically after some research I found this with that code as a solution for a (maybe) similar problem:

this.Context.Response.Write(response);
this.Context.Response.Flush();

May anyone explain to me what is going on there and how I can fix it (or modify the code of the other solution)?

EDIT1: I have ensured that the controller get's the information. So the error might be at the communication between controller and js. Any ideas?

EDIT2: Also adding

Response.BufferOutput = true;

at the Index() of the HomeController and the DBController doesn't change anything.

EDIT3: I identified that the Problem is with the JSON in the return. If I return a different type, it works fine. Any ideas why this happens and how to fix it?

EDIT4: The reason my code doesn't work is because the command

var result = dbcontext_hke.Database.SqlQuery<UnifiedResultset>(sql);

doesn't execute immediately I think. If I want to see what's inside result with Debug.WriteLine(result) I only get the plain sql command. Ideas?

Answer 1

Andrei was right - there was another line that caused this error. As stephbu mentioned in his comment as item 3: "the stream was unbuffered forcing the response headers to get written before the markup writing could begin."

This scenario was caused by the line

Response.Flush();

that was above the DB controller code I've mentioned