Question

I find myself using the ‘pattern’ below rather unsettlingly often, when I want to select entities with based only on the date part of a DateTime property. EF doesn’t parse the DateTime.Date property to T-SQL, so I end up using this code:

var nextDay = raceDate.Date.AddDays(1);
return EntityContext.RacingInfoes.SingleOrDefault(ri => ri.RaceDate >= raceDate && ri.RaceDate < nextDay);

It’s the most readable solution I have found so far, but I don’t like repeating it everywhere. However, I can’t encapsulate it in any method as that method is not recognised by the Linq to Entities parser. Is there anything I can do about this?

Answer 1

You can encapsulate it by writing a method like this:

Expression<Func<T, bool>> OnDate<T>(Expression<Func<T, DateTime>> selector,
                                    DateTime date)
{
    var nextDay = date.Date.AddDays(1);
    // Build up an expression tree, using Expression.AndAlso etc to
    // compare the result of applying the selector with both date and nextDay
}

Then you’d write:

return EntityContext.RacingInfoes.SingleOrDefault(Helper.OnDate(x => x.RaceDate),
                                                  raceDate);

(OnDate is a bad name, but you see what I mean…)