Why does LINQ query throw an exception when I attempt to get a count of a type

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public readonly IEnumerable<string> PeriodToSelect = new string[] { "MONTH" };  

var dataCollection = from p in somedata    
from h in p.somemoredate    
where h.Year > (DateTime.Now.Year - 2)    
where PeriodToSelect.Contains(h.TimePeriod)  
select new  
{  
    p.Currency, 
    h.Year.Month, h.Value                                                    
};

Can someone tell me why an exception is thrown when at the following line of code?

int count = dataCollection.Count();

This is the exception: 

System.NullReferenceException: Object reference not set to an instance of an object.
   at System.Linq.Enumerable.<SelectManyIterator>d__31`3.MoveNext()
   at System.Linq.Enumerable.<SelectManyIterator>d__31`3.MoveNext()
   at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at ...
exception-handling linq-to-objects linq c#
4 Answers

I ran into the same issue. It is annoying that MS did not provide built-in null detection and handling for the aggregate functions. The other issue is I wanted to ensure I got a 0 or $0 return result for nulls/empty query results, as I was working on dashboards/reporting. All of those tables would have data eventually, but early on you get a lot of null returns. After reading multiple postings on the subject I came up with this:

Retrieve the fields you want to return or later apply aggregate functions to first. Test for a null return. Return 0 if a null is detected.

If you do get actual data returned then you can safely utilize/apply the Count or Sum aggregate Linq functions. 

public ActionResult YourTestMethod()
{
    var linqResults = (from e in db.YourTable
                       select e.FieldYouWantToCount);

    if (linqResults != null)
    {
        return Json(linqResults.ToList().Count(), JsonRequestBehavior.AllowGet);
    }
    else
    {
        return Json(0, JsonRequestBehavior.AllowGet);
    }
}

Sum Example Below

public ActionResult YourTestMethod()
{
    var linqResults = (from e in db.YourTable
                       select e.Total);

    if (linqResults != null)
    {
        return Json(linqResults.ToList().Sum(), JsonRequestBehavior.AllowGet);
    }
    else
    {
        return Json(0, JsonRequestBehavior.AllowGet);
    }
}
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