r.a.d.combobox 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 autocomplete from SQL.

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