using System;

using System.Linq;

using System.Collections.Generic;

using System.Text.Json;

​

public class Program

{

    public static void Main()

    {

        // Student collection

        IList<employee> employees = new List<employee>() { 

            new employee() { employeeId = 1, employeeName = "John",month="1",salary="1000" } ,

            new employee() { employeeId = 1, employeeName = "John",month="2",salary="2000" } ,

            new employee() { employeeId = 1, employeeName = "John",month="3",salary="3000" } ,

            new employee() { employeeId = 1, employeeName = "John",month="4",salary="4000" } ,

        };

        var currentMonth=2;

        var Slarys = employees.Where(s => s.employeeId==1).ToList();

        var result=( from s in Slarys.Where(r=>r.month==currentMonth.ToString())                    

                    group s by s into results                   

                    select new {name=results.Select(a=>a.employeeName).SingleOrDefault(),

                               NewMonth=results.Select(a=>a.month=currentMonth.ToString()).SingleOrDefault(),

                               OldMonth=results.Select(a=>a.month=(currentMonth-1).ToString()).SingleOrDefault(),                             

                               salaryNew=results.Where(l=>l.month==currentMonth.ToString()).Select(a=>a.salary).SingleOrDefault(),

                               salaryOld=results.Where(l=>l.month==(currentMonth-1).ToString()).Select(a=>a.salary).SingleOrDefault()

                               }).ToList();

        var json = JsonSerializer.Serialize(result);

        Console.WriteLine(json);

​

    }

}

​

public class employee{

​

    public int employeeId { get; set; }

    public string employeeName { get; set; }

    public string month { get; set; }

    public string salary { get; set; }

}

​