Question

I’ve got a linq query that returns one item.
When I do myList.Select(p => p.ID) it throws null reference exception, however, if I do myList.First().ID it’s fine.
I have already tried myList.ToList().Select(p => p.ID) and failed as well.

The most bizarre thing is that it works on another pc…

Any idea?

Code update

   var test = A2012_DomainDB.GetGadgetDomainsForUser(userID);
   var viewableGadgetIDs = test.Where(p => p != null).Select(p => p.GadgetID); // this line fails 

public static IEnumerable<A2012_Domain_Gadget> GetGadgetDomainsForUser(int userID)
    {
        var db = Database_Factory.EVISION_EMAGINE_DB;
        var viewableDomainIDs = GetDomainsForUser(userID).Select(p => p.DomainID);
        var result = db.A2012_Domain_Gadget.Where(p => viewableDomainIDs.Contains(p.DomainID));
        return result;
    }

  public static List<A2012_Domain> GetDomainsForUser(int userID)
    {
        var db = Database_Factory.EVISION_EMAGINE_DB;
        List<int> viewableIDs = new List<int>();
        List<A2012_Domain> domains = new List<A2012_Domain>();

        viewableIDs.AddRange(db.A2012_Domain_User.Where(p => p.UserID == userID).Select(p => p.DomainID));                        

        viewableIDs.ForEach(i =>
        {
            domains.Add(db.A2012_Domain.Where(p => p.DomainID == i).Single());
        });

        return domains;
    }

Answer 1

I cannot really see exactly where this would come from. The GetGadgetDomainsForUser query should be deferred until you actually iterate the returned viewableGadgetIDs, but you seem to imply the exception occurs when just running that line.

All I can really suggest is to try null guarding all lambda expressions:

var result = db.A2012_Domain_Gadget
                 .Where(p => p != null)
                 .Where(p => viewableDomainIDs.Contains(p.DomainID));

And:

viewableIDs.AddRange(
    db.A2012_Domain_User
        .Where(p => p != null)
        .Where(p => p.UserID == userID)
        .Select(p => p.DomainID));                        

viewableIDs.ForEach(i =>
{
    domains.Add(
        db.A2012_Domain
           .Where(p => p != null)
           .Where(p => p.DomainID == i)
           .Single());
});

I’d be most tempted to say the error is coming from here:

var result = db.A2012_Domain_Gadget.Where(p => viewableDomainIDs.Contains(p.DomainID));

Perhaps the query into the db is yielding a null item, which is then used in p.DomainID without a null check.