Error during serialization or deserialization using JSON JavascriptSerializer

Changing the default maxJsonLength in web config won't work when trying to return a JsonResult, because asp.net will try to serialize your object internally while completely ignoring the maxJsonLenght you set.

One simple way to fix this, is to return a string instead of a JsonResult in your method, and instance a new JavaScriptSerializer with your custom maxJsonLenght, something like this:

 [HttpPost]
    public string MyAjaxMethod()
    {
        var veryBigJson = new object();

        JavaScriptSerializer s = new JavaScriptSerializer
        {
            MaxJsonLength = int.MaxValue
        };
        return s.Serialize(veryBigJson);
    }

And then in your view you just parse it back into a json with JSON.parse(data)

Another way would be creating your own JsonResult class when you can actually control the maxJsonLenght, something like this:

public class LargeJsonResult : JsonResult
    {
        const string JsonRequest_GetNotAllowed = "This request has been blocked because sensitive information could be disclosed to third party web sites when this is used in a GET request. To allow GET requests, set JsonRequestBehavior to AllowGet.";
        public LargeJsonResult(object data,JsonRequestBehavior jsonRequestBehavior=JsonRequestBehavior.DenyGet,int maxJsonLength=Int32.MaxValue)
        {
            Data = data;
            JsonRequestBehavior = jsonRequestBehavior;
            MaxJsonLength = maxJsonLength;
            RecursionLimit = 100;
        }

        public new int MaxJsonLength { get; set; }
        public new int RecursionLimit { get; set; }

        public override void ExecuteResult(ControllerContext context)
        {
            if (context == null)
            {
                throw new ArgumentNullException("context");
            }
            if (JsonRequestBehavior == JsonRequestBehavior.DenyGet &&
                String.Equals(context.HttpContext.Request.HttpMethod, "GET", StringComparison.OrdinalIgnoreCase))
            {
                throw new InvalidOperationException(JsonRequest_GetNotAllowed);
            }

            HttpResponseBase response = context.HttpContext.Response;

            if (!String.IsNullOrEmpty(ContentType))
            {
                response.ContentType = ContentType;
            }
            else
            {
                response.ContentType = "application/json";
            }
            if (ContentEncoding != null)
            {
                response.ContentEncoding = ContentEncoding;
            }
            if (Data != null)
            {
                JavaScriptSerializer serializer = new JavaScriptSerializer() { MaxJsonLength = MaxJsonLength, RecursionLimit = RecursionLimit };
                response.Write(serializer.Serialize(Data));
            }
        }
    }

And on your method you just change the type and return a new instance of the class

    [HttpPost]
    public LargeJsonResult MyAjaxMethod()
    {
        var veryBigJson = new object();
        return new LargeJsonResult(veryBigJson);
    }

Source: https://brianreiter.org/2011/01/03/custom-jsonresult-class-for-asp-net-mvc-to-avoid-maxjsonlength-exceeded-exception/

Can you do it like this in your code

JavaScriptSerializer serializer = new JavaScriptSerializer();
serializer.MaxJsonLength = Int32.MaxValue; 
myObject obj = serializer.Deserialize<yourObject>(yourJsonString);