Get default value type (DateTime) in Linq to Sql query with empty results

Question

I am having a problem returning a default DateTime value from a complex Linq-to-Sql query.

Hopefully the following simplified example shows the

Answer 1

I'd be very tempted to actually use a nullable DateTime for this. For example, from your "Car" sample:

var test = _dataContext.GetTable
                       .Select(c => 
                           c.MechanicVisits
                            .Select(m => m.ServiceRecord)
                            .Select(s => (DateTime?) s.ServiceDate)
                            .OrderByDescending(d => d)
                            .FirstOrDefault()
                       ).ToList();

That way I suspect you'll end up with it working and giving you null DateTime? values. You could always transform that later if you wanted:

var test = _dataContext.GetTable
                       .Select(c => 
                           c.MechanicVisits
                            .Select(m => m.ServiceRecord)
                            .Select(s => (DateTime?) s.ServiceDate)
                            .OrderByDescending(d => d)
                            .FirstOrDefault()
                       ).AsEnumerable()
                        .Select(dt => dt ?? DateTime.MinValue)
                        .ToList();

---

Original answer (doesn't work)

Hmm. I won't claim to fully understand the reasons for this, but here's a potential workaround:

users.Select(u =>
  new MyDomainObject(
     u.Id,
     u.Transactions
        .Where(t => false)
        .Select(t => t.TransactionTime)
        .OrderByDescending(x => x)
        .DefaultIfEmpty(DateTime.MinValue)
        .First()
  )
); 

In other words, if the result set is empty, use the specified default - and then take the first result of the now-definitely-not-empty sequence.