using System;

using System.Linq;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        NodeContainer nc = new NodeContainer();

        nc.Init();

        nc.Nodes[0] = new Node { Index = 0 };

        nc.Nodes[3] = new Node { Index = 3 };

        nc.Nodes[5] = new Node { Index = 5 };

​

        Console.WriteLine(nc.Encode());

    }

}

​

public class NodeContainer

{

    public Node[] Nodes { get; private set; }

    public void Init()

    {

        Nodes = new Node[9];

    }

    public string Encode()

    {

        return JsonConvert.SerializeObject(Nodes.Where(n => n != null), Formatting.None);

    }

}

​

public class Node

{

    public int Index { get; set; }

}

​