date difference with linq - linq

Question

With this code:
i.SpesaAlloggio = db.TDP_NotaSpeseSezB.Sum(p => p.Costo / (((DateTime)p.DayEnd)
.Subtract((DateTime)p.DayStart).Days + 1));
I receive this error:
LINQ to Entities does not recognize the method
'System.TimeSpan Subtract(System.DateTime)' method, and this method cannot be
translated into a store expression.
How can I do this?

Answer 1

Use a calculated DB field and map that. Or use SqlFunctions with EF 4 as LukLed suggested (+1).

Answer 2

I wrote a function for removing time:
public static DateTime RemoveHours(DateTime date)
{
int year = date.Year;
int month = date.Month;
int day = date.Day;
return new DateTime(year, month, day);
}
and changed filtering condition:
var query =
from trn in context.IdentityTransactions
where trn.ClientUserId == userId && trn.DateDeleted == null
orderby trn.DateTimeCreated
select new
{
ClientServerTransactionID = trn.ClientServerTransactionID,
DateTimeCreated = trn.DateTimeCreated,
ServerTransDateTime = trn.ServerTransDateTime,
Timestamp = trn.Timestamp,
Remarc = trn.Remarc,
ReservedSum = trn.ReservedSum,
};
if (dateMin.HasValue && dateMin.Value > DateTime.MinValue)
{
DateTime startDate = Converters.RemoveHours(dateMin.Value);
query = from trn in query
where trn.DateTimeCreated >= startDate
select trn;
}
if (dateMax.HasValue && dateMax.Value > DateTime.MinValue)
{
var endDate = Converters.RemoveHours(dateMax.Value.AddDays(1.0));
query = from trn in query
where trn.DateTimeCreated < endDate
select trn;
}
dateMin and dateMax are nullable types and may be not set in my case.

Answer 3

Try (it is not very efficient, but it will work):
i.SpesaAlloggio = db.TDP_NotaSpeseSezB.ToList()
.Sum(p => p.Costo / (((DateTime)p.DayEnd)
.Subtract((DateTime)p.DayStart).Days + 1));
EDIT : This will be extremely slow for large tables, because it transfers whole table content form server
Entity Framework tries to translate your expression to SQL, but it can't handle ((DateTime)p.DayEnd).Subtract((DateTime)p.DayStart). You have to make it simpler. ToList() gets all rows and then makes the calculation on application side, not in database.
With EF4, you could use SqlFunctions DateDiff
With EF1, you could create calculated field or view with this field and make calculation based on this field.