public class sampleClass
{
    public double x { get; set; }
    public double y { get; set; }
    public double z { get; set; }
    public double w { get; set; }
}

sampleClass c = new sampleClass()
{
    x = -1,
    y = 2,
    z = -1,
    w = 8
};

{
    "x": null,
    "y": "2",
    "z": null,
    "w": "8"
}

{
    "x": null,
    "y": "2",
    "z": "-1",
    "w": "8"
}

class DecimalConverter : JsonConverter
    {
        public override void WriteJson(JsonWriter jsonWriter, object inputObject, JsonSerializer jsonSerializer)
        {
            // Set the properties of the Json Writer
            jsonWriter.Formatting = Formatting.Indented;

            // Typecast the input object
            var numericObject = inputObject as double?;
            jsonWriter.WriteValue((numericObject == -1.0) ? null : numericObject);
        }

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            double? readValue = reader.ReadAsDouble();
            return (readValue == null) ? -1 : readValue;
        }

        public override bool CanConvert(Type objectType)
        {
            return (typeof(double) == objectType);
        }
    }