Creating a collection with custom Json serializer

Question

This code is part of a ASP.Net Web API controller, where I have a special requirement to modify certain values in the final Json, before sending it across to UI

My current code looks following:

// Web API controller

public partial class ViewRController
        {
       [HttpGet]
            public DataMapList FetchEmployeeDataList()
            {
                DataMapList returnDataMapList = new DataMapList();

                return (returnDataMapList);
            }
        }

// Type definition and details

    [JsonObject]
        [JsonConverter(typeof(DataMapListSerializer))]
        public class DataMapList
        {
            public List<Dictionary<object, object>> employeeSalaryMappingList = null;

            public DataMapList()
            {
                employeeSalaryMappingList = new List<Dictionary<object, object>>();
                employeeSalaryMappingList.Add(new Dictionary<object, object>());

                employeeSalaryMappingList[0].Add(1, 10000);
                employeeSalaryMappingList[0].Add(2, 13000);
                employeeSalaryMappingList[0].Add(3, 15000);

                employeeSalaryMappingList.Add(new Dictionary<object, object>());

                employeeSalaryMappingList[1].Add(4, 9000);
                employeeSalaryMappingList[1].Add(5, 12000);
                employeeSalaryMappingList[1].Add(6, 11000);
            }
        }

// Custom Json serializer

public class DataMapListSerializer : JsonConverter
    {
        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            var employeeSalaryDataMapList = value as DataMapList;
            var employeeSalaryMappingList = employeeSalaryDataMapList.employeeSalaryMappingList;


            writer.WriteStartObject();
            foreach (var employeeSalaryMappingDictionary in employeeSalaryMappingList)
            {
                writer.WritePropertyName("ED");
                writer.WriteStartObject();

                foreach (var keyValuePair in employeeSalaryMappingDictionary)
                {
                    writer.WritePropertyName("ID-" + keyValuePair.Key);
                    serializer.Serialize(writer, keyValuePair.Value);
                }

                writer.WriteEndObject();
            }

            writer.WriteEndObject();
        }       

        public override bool CanConvert(Type objectType)
        {
            return typeof(DataMapList).IsAssignableFrom(objectType);
        }
    }

Issue that I face is following is my result:

{ 
   "ED": { "ID-1": 10000, "ID-2": 13000, "ID-3": 15000 }, 
  "ED": { "ID-4": 9000, "ID-5": 12000, "ID-6": 11000 } 
}

However I expect the following result, since I am serializing a collection (.Net List type, .Net collection)

[ 
   "ED": { "ID-1": 10000, "ID-2": 13000, "ID-3": 15000 }, 
  "ED": { "ID-4": 9000, "ID-5": 12000, "ID-6": 11000 } 
]

In fact the current result when I try to validate in Json Lint , it shows valid Json but terminates a part of it,so it shows the following as valid Json, however that's just a diversion

{
    "ED": {
        "ID-4": 9000,
        "ID-5": 12000,
        "ID-6": 11000
    }
}

I have tried replacing writer.WriteStartObject(); and writer.WriteEndObject(); by writer.WriteStartArray(); and writer.WriteEndArray(); , before the first foreach loop that traverse through the List in the type, but that leads to the exception, so doesnt help

"exceptionMessage": "Token StartArray in state ObjectStart would result in an invalid JSON object. Path '

any suggestion to resolve the issue let me know if you need any clarification

Answer 1

In JSON , name-value pairs must be within an object {} ; they cannot be directly inside an array [] . So this JSON that you are trying to create is not valid, and that is why you are getting an exception:

[ 
   "ED": { "ID-1": 10000, "ID-2": 13000, "ID-3": 15000 }, 
   "ED": { "ID-4": 9000, "ID-5": 12000, "ID-6": 11000 } 
]

In order to make it a valid collection, each of the ED name-value pairs must be wrapped in an object like this:

[ 
   { 
      "ED": { "ID-1": 10000, "ID-2": 13000, "ID-3": 15000 } 
   }, 
   { 
      "ED": { "ID-4": 9000, "ID-5": 12000, "ID-6": 11000 } 
   }
]

Once you see that, the solution becomes clear: in your DataMapListSerializer you need to use writer.WriteStartArray() and writer.WriteEndArray() outside your loop, and you need to add writer.WriteStartObject() and writer.WriteEndObject() inside your loop.

    writer.WriteStartArray();

    foreach (var employeeSalaryMappingDictionary in employeeSalaryMappingList)
    {
        writer.WriteStartObject();

            writer.WritePropertyName("ED");
            writer.WriteStartObject();

            foreach (var keyValuePair in employeeSalaryMappingDictionary)
            {
                writer.WritePropertyName("ID-" + keyValuePair.Key);
                serializer.Serialize(writer, keyValuePair.Value);
            }

            writer.WriteEndObject();

        writer.WriteEndObject();
    }

    writer.WriteEndArray();

Fiddle: https://dotnetfiddle.net/OT4HKM