Dear Jovana,
'All' is a pretty big word ... here is a sample populating check box lists in .NET forms with some of them, at least:
Cheers,
Jeremy
Definitelly, I wanted to point that out too. I can not imagine anyone benefitting from having a combo box filled with ALL elements in a model. It would probably take forever to fill and it would be nearly impossible to display and scroll over the items. Besides, I have a feeling that combo-boxes have a limit on maximum items they may contain and the limit woudl likely be less than the number of elements typical Revit models have.
Thank you