The null value cannot be assigned to a member with type System.DateTime

Question

I have exception:

The null value cannot be assigned to a member with type System.DateTime which is a non-nullable value type. 
Source Error: 
var bd = (from d in baza.hpurs where d.userID == userID && d.date !=null select d.date).Max();

My method:

 public ActionResult Add_hours(int userID)
    {
        var dat = DateTime.Today;

        var bd = (from d in baza.hours where d.userID == userID && d.date !=null select d.date).Max();

        if (bd != dat)
        {
            return View();
        }

     else
    {
        var dd = (from d in baza.hours where d.userID == userID && d.date == dat select d.hoursID).Single();
        return RedirectToAction("hours_is", "Employer", new { HoursID = dd });
    }
}

It works good, but only when I have 1 or more data in hours table for concrete user. This error is when I want add hours to table where concrete user hasn't added any hours. I haven't any idea how fix it...

Answer 1

This is one of the points where LINQ to SQL is a leaky abstraction. The semantics of the Max() function in C# is to throw an InvalidOperationException if there are no elements in the sequence. The semantics of SQL's MAX() function is to return NULL. The C# code compiles as if the C# semantics are followed, but the code is never executed as C# code. It is translated into SQL where the SQL semantics rules.

To handle this, you need to decide what you want when there are no matching elements. If you want a null value, explicitly declare a nullable variable and introduce an extra cast to DateTime? to tell LINQ to SQL that null might be returned.

DateTime? bd = (from d in baza.hpurs 
                where d.userID == userID && d.date !=null 
                select (DateTime?)d.date).Max();