using System;

using Newtonsoft.Json;

using Newtonsoft.Json.Serialization;

public class Program

{

    public static void Main()

    {

        var viewModel = new ViewModel()

        { 

            UserMarket = "user_market", 

            PriceDiscount = 0.3M, 

            CollectionID = "all"

        };

        var json = JsonConvert.SerializeObject(viewModel);

        Console.WriteLine(json);

    }

}

​

public class ViewModel

{

    public string UserMarket { get; set; }

    public decimal PriceDiscount { get; set; }

    [JsonConverter(typeof(JavaScriptValueConverter))]

    public string CollectionID { get; set; }

}

​

public class JavaScriptValueConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        return objectType == typeof(string);

    }

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

    {

        return reader.Value;

    }

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

    {

        writer.WriteRawValue((string)value);

    }

}