using System;

using System.Linq;

using Newtonsoft.Json;

​

namespace ConsoleApplication1

{

    public class Program

    {

        public static void Main(string[] args)

        {

            var column = new Column

            {

                C1 = "Some text",

                C2 = "22",

                C3 = "{\"Id\":12,\"Name\":\"a name\"}",

            };

            string[] columnsWithJson = { "C3" };  //I know which columns that already contains json

            var json = JsonConvert.SerializeObject(column, new MyJsonConverter(columnsWithJson));

            Console.WriteLine(json);

        }

    }

​

    public class Column

    {

        public string C1 { get; set; }

        public string C2 { get; set; }

        public string C3 { get; set; }

    }

    public class MyJsonConverter : JsonConverter

    {

        string[] columnsWithJson;

​

        public override bool CanRead { get { return false; } }

​

        public MyJsonConverter(string[] columnsWithJson)

        {

            this.columnsWithJson = columnsWithJson;

        }

​

        public override bool CanConvert(Type objectType)

        {

            return objectType.Equals(typeof(String));

        }

​

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)

        {

            return null;  //no need, we are setting canRed to false. 

        }

​

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)

        {

            if(columnsWithJson.Contains(writer.Path))

                writer.WriteRawValue((String)value);    

            else

                writer.WriteValue(value);

        }

    }

}