>I trie to create a collection in a session based class; but this is not possible; I get an error.
>
>Could someone explain why?

Yes. The Session base class is designed for lightweight encapsulations of private datasessions. To achieve this, it does not support containership (ADD OBJECT, Addobject(), ...). If you need both private datasessions and containership, use Form instead. Ex:

DEFINE CLASS PrivateWithCollection AS Form
  DataSession = 2  && private DS
  ADD OBJECT SomeCollection AS Collection
ENDDEFINE

-- Randy