Question

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 ...

Answer 1

The exception is thrown at the Count() statement because LINQ uses deferred execution and the actual LINQ query will not be executed until to call .Count(), .ToList(), etc.