Combobox: Property-Caption different to Container-Caption


i need a combobox that uses a different caption for its items wheter the item is displayed in the dropdown-selection or as the selected item.
When displaying an item in the dropdown list, i want to show a shorter caption. But when this item gets selected and the combobox gets closed, the full caption should get displayed.

How can i achieve this behaviour?

I tried this on my own for a while and didn’t succeeded.
It seems that the caption is not recalculated for the “closed” state of the dropdown-box.

(I tried to change the caption-mode and use the toString-method of some proxy-Items and force the combobox to replaint. )

Thanks for your help.