using System;

using System.Linq;

​

enum EventType

{

   SessionStart,

   SessionEnd

}

​

record UserEvent(EventType EventType, int UserId, int TimeStamp);

​

public class Program

{

   public static void Main()

   {

      var events = new UserEvent[] {

         new(EventType.SessionStart, 1, 0001),

         new(EventType.SessionEnd, 1,   0002),

         new(EventType.SessionStart, 2, 0003),

         new(EventType.SessionEnd, 2,   0004),

         new(EventType.SessionStart, 3, 0005),

         new(EventType.SessionEnd, 3,   0006),

         new(EventType.SessionStart, 1, 0007),

         new(EventType.SessionStart, 2, 0008),

         new(EventType.SessionEnd, 1,   0009),

         new(EventType.SessionEnd, 2,   0010),

      };

​

      var wantedUserIds = new int[] { 1, 2 };

​

      var recentLoginsOriginal = events

         .Where(e => e.EventType == EventType.SessionStart && wantedUserIds.Contains(e.UserId))

         .OrderByDescending(e => e.TimeStamp)

         .DistinctBy(e => e.UserId);       

      var recentLoginsNew = events

          .Where(e => e.EventType == EventType.SessionStart && wantedUserIds.Contains(e.UserId))

          .GroupBy(e => e.UserId)

          .Select(eGroup => eGroup.OrderByDescending(e => e.TimeStamp).First());

​

      Console.WriteLine(string.Join("\n", recentLoginsOriginal));

      Console.WriteLine();

      Console.WriteLine(string.Join("\n", recentLoginsNew));

   }

}

​