title: LINQ Join to find items NOT IN a list
body: <p>How do I use LINQ across Queryable and non-Queryable data?</p>
  
  <p>With the following code I want to end up with a list of unselected users, i.e. everyone in allUsers that is not in selected.</p>
  
  <pre><code>public IQueryable&lt;CompanyUser&gt; FindAllUsersNotAssignedTothisCompany(int companyID)
  {
      var allUsers = Membership.GetAllUsers();
      var selected = db.CompanyUsers.Where(c =&gt; c.CompanyID == companyID);
      ...?
  }
  </code></pre>
  
  <p>I effectively want a list of User ID's and Names suitable for use in a DropDownList (ASP.NET MVC)</p>