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ID: 1 Name: Arnold; BirthDay: 15.06.2020 00:00:00
ID: 2 Name: Stefan; BirthDay: 15.06.2020 00:00:00

class Program
    {
        public class Student
        {
            public int ID { get; set; }
            public int StandardID { get; set; }
            public string Name { get; set; }
            public DateTime Birthdate { get; set; }
        }

        static void Main(string[] args)
        {
            List<Student> studentList = new List<Student>() {
                new Student() { ID = 1, Name = "Arnold", Birthdate = new DateTime(2020, 06, 15)},
                new Student() { ID = 1, Name = "Arnold", Birthdate = new DateTime(2020, 01, 01)},
                new Student() { ID = 2, Name = "Stefan", Birthdate = new DateTime(2020, 06, 15)},
                new Student() { ID = 2, Name = "Stefan", Birthdate = new DateTime(2020, 06, 01)},
            };
            
            int[] Ids = { 1, 2, 3 };

            List<Student> students = studentList
                .Where(s => Ids.Contains(s.ID) )
                .GroupBy(s => s.ID)
                .SelectMany(g => g.OrderByDescending(s => s.Birthdate) ).ToList();

            foreach (var i in students)
            {   
                Console.WriteLine($"ID: {i.ID} Name: {i.Name}; BirthDay: {i.Birthdate} ");
            }
            Console.ReadKey();
        }

ID: 1 Name: Arnold; BirthDay: 15.06.2020 00:00:00
ID: 1 Name: Arnold; BirthDay: 01.01.2020 00:00:00
ID: 2 Name: Stefan; BirthDay: 15.06.2020 00:00:00
ID: 2 Name: Stefan; BirthDay: 01.06.2020 00:00:00

var mytotal = _context.Apiapplicant.Where(c => !c.ApiRequestDate.Equals("") && c.IsDeleted.Equals(false)).GroupBy(o => new
            {
                Month = o.ApiRequestDate.Substring(5, 2),
                Year = o.ApiRequestDate.Substring(0, 4)
            }).Select(g => new
            {
                Month = g.Key.Month,
                Year = g.Key.Year,
                Total = g.Count()
            }).OrderByDescending(a => a.Year).ThenByDescending(a => a.Month).ToList();

var numerator = from t1 in _context.Apiapplicant
                            join t2 in _context.ApiApplicantHistory on t1.Id equals t2.ApiApplicantId
                            join t3 in _context.EntityType on t2.LastReqStatus equals t3.Id
                            //join t4 in mytotal on new { t2.Date.Substring(0,4), t2.Date.Substring(2,5) } equals new { t4.Year, t4.Month }
                            // join t4 in mytotal on  t2.Date.Substring(2, 5) equals t4.Month

                            where t1.IsDeleted == false && t3.Name == "granted" && t2.Date != null && t1.ApiRequestNo != null

                            group t1
                            by new
                            {
                                lastReq = t2.LastReqStatus,
                                Year = t2.Date.Substring(0, 4),
                                Month = t2.Date.Substring(5, 2)
                            } into g

                            select new
                            {
                                Year = g.Key.Year,
                                Month = g.Key.Month,
                                lastReq = g.Key.lastReq,
                                GrantedCount = g.Count()
                            };


            var GrantedReqStatus = numerator.OrderByDescending(x => x.Year).ThenByDescending(a => a.Month).ToList();

var LastGrantedStatus = (from t1 in mytotal
                                    from t2 in GrantedReqStatus
                                    where (t1.Month == t2.Month) && (t1.Year == t2.Year)
                                    group t1
                            by new
                            {
                                //  lastReq = t2.LastReqStatus,
                                Year = t1.Year,
                                Month = t1.Month
                            } into g
                                    select new {  Month=g.Key.Month, Year=g.Key.Year,grcount = g.Count()}).ToList();