RadComboBox supports autocomplete functionality - you can set a poll-server timeout and the combobox fires a server-side event that returns all the relative matches for the currently typed text. You can easily select an entry or re-request more entries depending and the ShowMoreResultsBox property. This example shows auto-complete from a flat text dictionary file.

Setting the AllowCustomText property to false will not allow typing of text that does not exist in the items collection of the treeview.