using System;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var parameter = new Parameter

        {

            defaults = new List<object>

            {

                100, 350, 550, 800, 850, 1000, 1050, 1150, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, 2100, 2200, 2350, 2400, 2500, 2550

            }

        };

​

        var settings = new JsonSerializerSettings

        {

            Converters = new List<JsonConverter> { new CustomFormattingArrayConverter(10) },

            Formatting = Formatting.Indented

        };

​

        var json = JsonConvert.SerializeObject(parameter, settings);

        Console.WriteLine(json);

    }

}

​

public class Parameter

{

    public List<object> defaults { get; set; }

}

​

public class CustomFormattingArrayConverter : JsonConverter

{

    private int ItemsPerLine { get; set; }

​

    public CustomFormattingArrayConverter(int itemsPerLine)

    {

        if (itemsPerLine < 1) throw new ArgumentException("itemsPerLine must be 1 or more");

        ItemsPerLine = itemsPerLine;

    }

​

    public override bool CanConvert(Type objectType)

    {

        return objectType == typeof(List<object>);

    }

​

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

    {

        List<object> list = (List<object>)value;

        writer.WriteStartArray();

        for (int i = 0; i < list.Count; i++)

        {

            if (i % ItemsPerLine == 0) writer.Formatting = Formatting.Indented;

            serializer.Serialize(writer, list[i]);

            if (i % ItemsPerLine == 0) writer.Formatting = Formatting.None;

        }

        writer.Formatting = Formatting.Indented;

        writer.WriteEndArray();

    }

​

    public override bool CanRead

    {

        get { return false; }

    }

​

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

    {

        throw new NotImplementedException();

    }

}

​