Hierarchical object retrieve childsIds with linq

How can I do this in one linq statement: If got an list of objects where I have (id, name, parentId). Now I have 3 levels of hierarchy in this list. What I want is that when I'm getting the Id of the top level in my function, I want the Ids of the 3th level.

I've got this for the moment:

Public List<int> GetIds(int objectId){
   var idList = new List<int>();
   //get the seconds level ids 
   var parentsIds = from n in myListObject
          开发者_如何学JAVA          where n.parentId.equals(objectId)
                    select n.id;
   //get the third level ids
   foreach(var parentId in parentsIds){
     var childIds = from c in myListObject
                    where c.parentId.equals(parentId)
                    select c.id;
     foreach(var childId in childIds){
       idList.Add(childId);
     }
   }
   return idList;
}

So I'm wondering if I could do this in one linq statement?

I have not tested it, but something similar to this should work:

myListObject
  .Where(child =>
     myListObject
     .Where(obj => obj.parentId==objectId)
     .Select(obj => obj.id)
     .Contains(child.parentId))
  .Select(child => child.Id);