I repeatedly hear from developers who wish to define a specific enumerated set of parameter values for their add-ins and limit the selection to these values in the element property palette user interface. Internally, Revit does implement a system to handle this, for instance by using negative element ids for element property drop-down list enumerations. This has also been a long-standing wish list item, and unfortunately still remains in that state, currently incorporated in the issue CF-3498 API wish: drop-down enumeration parameters for combo box... Read more →