using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var myList= new List<BtnCountViews>

        {

            new BtnCountViews{ DayOfYear = 5, BtnCount = 10, Views = 5},

            new BtnCountViews{ DayOfYear = 25, BtnCount = 8, Views = 6},

            new BtnCountViews{ DayOfYear = 52, BtnCount = 12, Views = 7},

            new BtnCountViews{ DayOfYear = 75, BtnCount = 15, Views = 8},

            new BtnCountViews{ DayOfYear = 250, BtnCount = 20, Views = 9}

        };

        var filled = GetBackFilledList(myList);

        filled.OrderBy(f => f.DayOfYear).Dump();

    }

    public static IList<BtnCountViews> GetBackFilledList(IList<BtnCountViews> incoming)

    {

        var map = incoming.ToDictionary(k => k.DayOfYear, v => v);

        var defaultList = GetDefaultList();

        foreach(var itm in defaultList)

        {

            if (map.ContainsKey(itm.DayOfYear)) continue;

            map.Add(itm.DayOfYear, itm);

        }

        return map.Select(m => m.Value).ToList();

    }

    public static IList<BtnCountViews> GetDefaultList()

    {

        //var daysInYear = DateTime.IsLeapYear(DateTime.Today.Year) ? 366 : 365;

        var defaultList = Enumerable.Range(1, 365).Select(e =>

            new BtnCountViews

            {

                DayOfYear = e,

                BtnCount = 0,

                Views = 0

            }

        ).ToList();

        return defaultList;

    }

    // Define other methods and classes here

    public class BtnCountViews

    {

        public int DayOfYear { get; set; }

        public int BtnCount { get; set; }

        public int Views { get; set; }

    }   

}