c# 4.0 - Linq Invalid Cast Exception Same Object Type -

I have written this question and as I have improved my understanding of business rules, I have modified it.

In this most recent visit, I was testing to see that in fact I have some redundancy which can be removed. Let me query you first error.

  Public listing & lt; ExternalForums & gt; GetAllExternalForums (int extforumBoardId) {list & lt; ExternalForums & gt; Xtrnlfrm = new list & lt; ExternalForums & gt; (); Var query = _forumExternalBoardsRepository.Table. Where (id = & gt; id.Id == extoformBoardId). Select (ExtForum = & gt; ExtForum.ExternalForums); Foreign currency (external forum item in query) {xtrnlfrm.Add (new external forum {id = item.id, forum-name = item.forname, forum url = item.form url}); } Return xtrnlfrm; }   
 It is not clear now that the query is returning the list of ExternalForums. I then add loop and item to the list of other fororphorphis objects through the list. This is redundancy which was hoping to remove me.  
 The precompiler was gtg, so I was kosher before re-once again and went as a strange error as I started the loop.  
 
 Unable to remove system object. To type Colllections.Generic.HashSet NamSpcA.NamSpcB.ExternalForums, NamSpcA.NamSpcB.ExternalForums   
 huh? They are the same object type.  
 Am I doing something wrong in my way of presenting the selection?  
 TIA   
 
 
  var query = _forumExternalBoardsRepository.Table. Where (id = & gt; id.Id == extoformBoardId). Select (ExtForum = & gt; ExtForum.ExternalForums);   
 This query returns  IEnumerable & lt; T & gt;  where  T  is type property, which is expected to be another collection, this time the  ExternalForum  and the error message Matches that, you have  IEnumerable & lt; HashSet & lt; ExternalForums & gt; & Gt; .  
 If you have a large collection of archive archives <> external forums  use  SelectMany  instead:  
  var query = _forumExternalBoardsRepository .Table. Where (id = & gt; id.Id == extoformBoardId). Selection (many) ExtForum = & gt; ExtForum.ExternalForums);