Question

my code
I need to grab the bottom element (most recent) entry into my database and see if it was over 1 hour ago, the type of ObservationTime is DateTime. I keep getting an error saying that “Sequence contains more than one element” for my var mWeathers. I think it has to do with the way I’m orderby descending but i cant figure it out. Thanks

    [OperationContract]
    public bool LeastOneHour()
    {

        DataClassesDBDataContext db = new DataClassesDBDataContext();
        var mWeathers = (from weathertable in db.WeatherTables 
                         orderby weathertable.ObservationTime descending 
                         select weathertable.ObservationTime).Single();
        DateTime lastTime = Convert.ToDateTime(mWeathers).AddHours(1);
        if ( lastTime <= DateTime.Now)
            return true;
        else
            return false;
    }

Answer 1

Single() says there must be one element in the result, and if there are more than one, throw an exception. What you’re likely looking for is First(), or FirstOrDefault() if there’s a chance there will be no results at all (in which case null is returned)

If you can be guaranteed that there will always be an element, try this:

DateTime lastDate = db.WeatherTables.OrderByDescending (w => w.ObservationTime).Select(w => w.ObservationTime).First();

return lastDate.AddHours(1) <= DateTime.Now;

Otherwise something like

DateTime? lastDate = db.WeatherTables.OrderByDescending (w => w.ObservationTime).Select(w => w.ObservationTime).FirstOrDefault();

return lastDate.HasValue ? lastDate.AddHours(1) <= DateTime.Now : false;

Assuming of course ObservationDate is declared as DateTime?