using System;

using System.Collections.Generic;

using System.Linq;

using System.Globalization;

public class Program

{

    public class Foo

    {

        public string id;

        public DateTime date;

    }

    public static void Main()

    {

        List<Foo> cv = new List<Foo>(){     

                new Foo(){id = "01200", date = DateTime.ParseExact("11/11/2020", "dd/MM/yyyy", CultureInfo.InvariantCulture)},            

                new Foo(){id = "01200", date = DateTime.ParseExact("11/11/2021", "dd/MM/yyyy", CultureInfo.InvariantCulture)},            

                new Foo(){id = "02100", date = DateTime.ParseExact("01/01/2019", "dd/MM/yyyy", CultureInfo.InvariantCulture)},

                new Foo(){id = "02100", date = DateTime.ParseExact("01/01/2029", "dd/MM/yyyy", CultureInfo.InvariantCulture)}

        };

        //  OPTION 1 :

        Foo res1 =  (from c in cv

                     join c2 in

                          (from c3 in cv

                           where !(c3.id == null || c3.id.Trim() == string.Empty)

                           group c3 by c3.id into cgroup

                           select new

                           {

                              ID = cgroup.Key,

                              MaxDate = cgroup.Max(x => x.date),

                           }

                          ) on new { JoinProperty1 = c.id, JoinProperty2 = c.date } equals

                               new { JoinProperty1 = c2.ID, JoinProperty2 = c2.MaxDate } 

                    select c).Aggregate((u,v) => u.date < v.date ? u: v);       

         res1.Dump();       

         // OPTION 2 :  

         var res2 = cv.GroupBy(

                         p => p.id, 

                         p => p.date,

                         (key, g) => new { id = key, date = g.Max() }).ToList<Foo>().Aggregate((u,v) => u.date < v.date ? u: v);

         res2.Dump();   

    }

}