C# how to group records by timestamp?

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C#

I am using C# NHibernate to create a dashboard and what I am trying to do is group the records by timestamp so if there are identical timestamps it will only show once. I have tried Last(t => t.timestamp) but receive the error of cannot convert type System.DateTime to bool and when I tried .GroupBy(t => t.timestamp) I get <IGrouping<DateTime, Answer> does not contain a definition for my fields of eventId, userId, and timestamp.



Open to suggestions. Thanks

What I have tried:

public ActionResult Dashboard()
        {
            using (ISession session = OpenNHibernateSession.OpenSession())
            {

               /*These answers gets my list of data. Here is where I am trying 
                 to use a GroupBy clause so records with identical timestamps
                 will only be shown once.*/
                var answers = session.Query<Answer>().OrderByDescending(t => t.timestamp).ToList();
               
                var dashboardVms = new List<DashboardViewModel>();
                foreach (var answer in answers)
                {
                    var dashboardVm = new DashboardViewModel
                    {
                        EventName = session.Get<Event>(answer.eventId, LockMode.Read).name,
                        EventId = answer.eventId,
                        UserId = answer.userId,
                        TimeStamp = answer.timestamp
                    
                    };
                    dashboardVms.Add(dashboardVm);
                }
                
                return View(dashboardVms);
            }
             
        }