using System;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var ar = new Hoge[10];

        Array.Fill<Hoge>(ar, new Hoge{x = 1, y = 0.1});

        System.Console.WriteLine(JsonConvert.SerializeObject(ar));

    }

}

​

public class Hoge{

    public int x;

    public double y;

}