title: LINQ Query Except not working, List<long?> vs. List<long>
body: <p>What I'm trying to do is narrow the results of a query that I will use to databind later on. I want to get all of the ProgramIds that are used in my gridview, and remove them from the datasource of my dropdownlist (ie, so that a user cannot choose to create and insert an object into the gridview of the same ProgramId as one that already exists)</p>
  
  <p>Here's some code:</p>
  
  <pre><code>var query = from goals in DataContext.Goals
              select goals;
  var query2 = (from goals in query.AsEnumerable()
                select goals.ProgramId).ToList(); //List&lt;long?&gt;
  var query3 = (from progs in DataContext.Programs
                select progs.ProgramId).ToList(); //List&lt;long&gt;
  var cgps = query3.Except(query2);
  </code></pre>
  
  <p>And I'm getting these errors on <code>var cgps = query3.Except(query2);</code> :</p>
  
  <blockquote>
    <p>Error 29  'System.Collections.Generic.List' does not contain a definition for 'Except' and the best extension method overload 'System.Linq.ParallelEnumerable.Except(System.Linq.ParallelQuery, System.Collections.Generic.IEnumerable)' has some invalid arguments  C:...\Shmeh\Shmeh\Shmeh\this.aspx.cs   24  226 Project</p>
    
    <p>Error 30  Instance argument: cannot convert from 'System.Collections.Generic.List' to 'System.Linq.ParallelQuery'    C:...\Shmeh\Shmeh\Shmeh\this.aspx.cs   24  226 Project</p>
  </blockquote>
  
  <p>If you have any idea how to validly do what I'm trying to do, any help would be greatly appreciated! Thanks!</p>