Multiple Select:

RadComboBox supports client-side autocomplete - end users can now autocomplete combobox items that have been loaded on the client without the need of callbacks and handling the ItemsRequested server-side event. The only thing you need to do is to set the MarkFirstMatch property to true and RadComboBox will automatically autocomplete all combo items. You can also autocomplete multiple items using any delimiter - you can set the AutoCompleteSeparator property to any delimiter character (e.g. ";", ",", etc)

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