WHERE clause on null in linq

Question

I have two tables which, to simplify, look like this:

TheFiles

FileID | UserID
 23    |   342
 53    |   352

TheData

UserID |  DateCreated
  352  |    7/22/2014
  245  |    7/25/2014  
  589  |    7/28/2014

I'm looking to return a nullable int that represents the latest UserID from the table TheData that's not in the table TheFiles . So for example, with this sample data, the return value would be 589 because it's the latest entry in TheData and it's not in the table TheFiles.

I'm having some trouble on the where when there's no data. This is what I have:

var TheNullableInt = (from d in MyDC.TheData
                      where d.UserID doesn't exist in MyDC.TheFiles
                      orderby d.DateCreated descending
                      select d.UserID).FirstOrDefault();

I'm using linq-to-SQL. How can I do the where clause?

Answer 1

var lastest = MyDC.TheData.Where(d => !MyDC.TheFiles.Any(f => f.UserID == d.UserID))
                          .OrderByDescending()
                          .FirstOrDefault();

Or if you really want to use LINQ

var latest = (from d in MyDC.TheData
              where !MyDC.TheFiles.Any(f => f.UserID == d.UserID)
              orderby d.DateCreated descending
              select d
             ).FirstOrDefault();

Answer 2

from d in MyDC.TheData
     where  !MyDC.TheFiles.Any(tf=>tf.UserID == d.UserID)

or do a join

from d in MyDC.TheData
join tf in MyDC.TheFiles on tf.UserID equals tf.UserID into j
from x in j.DefaultIfEmpty()
where x == null
select.....