using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        string json = @"

            {

                ""EighteenSigFigs"" : 9999999999999999.99,

                ""SeventeenSigFigs"" : 999999999999999.99,

                ""SixteenSigFigs"" : 99999999999999.99,

                ""FifthteenSigFigs"" : 9999999999999.99

            }";

        JsonSerializerSettings settings = new JsonSerializerSettings();

        settings.Converters.Add(new DecimalJsonConverter());

        settings.Formatting = Formatting.Indented;

        Foo foo = JsonConvert.DeserializeObject<Foo>(json, settings);

        Console.WriteLine("EighteenSigFigs: " + foo.EighteenSigFigs);

        Console.WriteLine("SeventeenSigFigs: " + foo.SeventeenSigFigs);

        Console.WriteLine("SixteenSigFigs: " + foo.SixteenSigFigs);

        Console.WriteLine("FifthteenSigFigs: " + foo.FifthteenSigFigs);

        Console.WriteLine();

        json = JsonConvert.SerializeObject(foo, settings);

        Console.WriteLine(json);

    }

}

​

class Foo

{

    public decimal EighteenSigFigs { get; set; }

    public decimal SeventeenSigFigs { get; set; }

    public decimal SixteenSigFigs { get; set; }

    public decimal FifthteenSigFigs { get; set; }

}

​

public class DecimalJsonConverter : JsonConverter

{

    public override bool CanConvert(Type objectType)

    {

        return (objectType == typeof(decimal));

    }

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

    {

        if (reader.TokenType == JsonToken.String)

        {

            string value = (string)reader.Value;

            if (value == string.Empty)

            {

                return decimal.MinValue;

            }

        }

        else if (reader.TokenType == JsonToken.Float || 

                 reader.TokenType == JsonToken.Integer)

        {

            return Convert.ToDecimal(reader.Value);

        }

        throw new JsonSerializationException("Unexpected token type: " + reader.TokenType.ToString());

    }

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

    {

        decimal dec = (decimal)value;

        if (dec == decimal.MinValue)

        {

            writer.WriteValue(string.Empty);

        }

        else

        {

            writer.WriteValue(dec);

        }

    }

}

​