RadDropDownList supports a number of client-side events that let you customize the behavior of the dropdownlist:
OnClientDropDownOpening occurs before the drop-down list opens.
OnClientDropDownOpened occurs after the drop-down list has been opened.
OnClientItemSelecting occurs before the item's selection.
OnClientItemSelected occurs after the item is selected.
OnClientSelectedIndexChanged occurs after the SelectedIndex has been changed in response to a user action.
OnClientDropDownClosing occurs before the drop-down list closes.
OnClientDropDownClosed occurs after the drop-down has been closed.
OnClientLoad occurs after the RadDropDownList has been fully initialized on the client-side.
OnClientItemDataBound occurs for each item that is created during Web Service(including if ODataSource control is used) binding.