RadComboBox has a client method called set_text(). I have just tested it on our online demos
and it works as expected. Whilst, the RadListBox does not have such client method -- you are allowed to only change the text of the items in the RadListBox.
Here is an example:
Would you please paste here the code involved in the troubled behavior you experience at your end?
All the best,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed