Question

I am on framework 4.0. This is a WPF application, on sql server CE, which is a little limiting.
I have an entity that looks like this:

    public class TimeEvent
    {
       public int Id
       {get; set;}
       public DateTime EventDate
       {get; set;}
       public bool CheckIn
       {get; set;}
    }

I have two types of events, one for checkin (CheckIn is true) and one for checkout (CheckIn is false). Each of the events happens once per day.
What I want to do using linq to entities, is to end up with a set of objects like this:

public class Diff
{
   public DateTime Date //The date of both events
   {get; set;}
   public DateTime CheckInTime //Time of first event
   {get; set;}
   public DateTime CheckOutTime //Time of second event
   {get; set;}
   public int Hours //Difference in hours.
   { get { return (CheckOutTime - CheckInTime).Hours;} 
}

There are validation rules in place so no more than one event of each type can happen in one day.
I tried using the aggregate function, but I’m really not getting anywhere.
Thank you!

Answer 1

Group your time events by EventDate date part. To get that you need to use EntityFunctions.TruncateTime method (simple EventDate.Date will not work with Entity Framework).

var query = from t in db.TimeEvents
            group t by EntityFunctions.TruncateTime(t.EventDate) into g
            let checkIn = g.Min(x => x.EventDate)
            let checkOut = g.Max(x => x.EventDate)           
            select new Diff {
                Date = g.Key,
                CheckInTime = checkIn,
                CheckOutTime = checkOut                    
            };

Also you can calculate time difference on server side with EntityFunctions.DiffHours(checkOut, checkIn).