using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var model = new myModel()

        {

            myString = "foo,bar,baz",

            myNumber = 21

        };

        Console.WriteLine("-- Serialize --");

        string json = JsonConvert.SerializeObject(model);

        Console.WriteLine(json);

        Console.WriteLine("\n-- Deserialize --");

        model = JsonConvert.DeserializeObject<myModel>(json);

        Console.WriteLine(model.myString);

        Console.WriteLine(model.myNumber);

    }

}

​

public class myModel

{

    [JsonConverter(typeof(CommaSeperatedStringJsonConverter))]

    public string myString { get; set; }

    public int myNumber { get; set; }

}

​

public class CommaSeperatedStringJsonConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        return objectType == typeof(string);

    }

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

    {

        var stringArray = serializer.Deserialize<string[]>(reader);

        var str = string.Join(",", stringArray);

        return str;

    }

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

    {

        if (value == null)

        {

            writer.WriteNull();

        } 

        else

        {

            serializer.Serialize(writer, value.ToString().Split(','), typeof(string[]));

        }

    }

}