title: .Except<T> is throwing an Exception
body: <p>I have the following code:</p>
  
  <pre><code>Public Shared Function GetAvailableManufacturers() As List(Of Manufacturer)
      'first get all the live orders and extract their mfrs'
      Dim sos As List(Of OrderForm) = GetFormsByStatus(StockStatus.Building)
      Dim unavailableMfrs As New List(Of Manufacturer)
      For Each so As StockingOrder In sos
          unavailableMfrs.Add(so.Source)
      Next
  
      'then remove all mfrs with an open SO from a list of all mfrs'
      Dim allMfrs As List(Of Manufacturer) = Manufacturer.GetManufacturers
      Return allMfrs.Except(unavailableMfrs) &lt;----- error here
  End Function
  </code></pre>
  
  <p>Explanation of what the above does:</p>
  
  <ul>
  <li><code>GetFormsByStatus()</code> self-explanatory</li>
  <li><code>GetManufacturers()</code> returns a list of all manufacturers in the database </li>
  </ul>
  
  <p>My idea to get available manufacturers was to get a list of all the manufacturers in my open forms, then get a list of all manufacturers and exclude all the manufacturers in the first list, illustrated like so (pseudo):</p>
  
  <pre><code>List A: {1,2,3,4,5,6,7,8,9,10}
  List B: {5,7,10}
  Result: {1,2,3,4,6,8,9}
  </code></pre>
  
  <p>I have set up my <code>Manufacturer</code> class according to <a href="http://christopherdeweese.com/blog2/post/loving-linq-generics-using-except-with-equalitycomparert" rel="nofollow noreferrer">this article</a> so that it can be compared, but I'm still getting this error:</p>
  
  <blockquote>
    <p>Unable to cast object of type '<code>&lt;ExceptIterator&gt;d__92'1[csCore.Manufacturer]'</code> to type '<code>System.Collections.Generic.List'1[csCore.Manufacturer]</code>'.</p>
  </blockquote>
  
  <p>I thought at first that because during testing <code>GetFormsByStatus()</code> returns 0 results maybe that was causing problems, but it doesn't make sense that <code>Except()</code> wouldn't work if the provided list had 0 items. Can anyone spot what I'm doing wrong?</p>
  
  <p>Thanks so much!</p>