Implement this method to provide control over whether your external command is enabled or disabled.
Assembly: RevitAPIUI (in RevitAPIUI.dll) Version: 2015.0.0.0 (2015.0.0.0)
An ApplicationServices.Application object which contains reference to Application needed by external command.
An list of categories of the elements which have been selected in Revit in the active document, or an empty set if no elements are selected or there is no active document.
Return ValueIndicates whether Revit should enable or disable the corresponding external command.
This callback will be called by Revit's user interface any time there is a contextual change. Therefore, the callback must be fast and is not permitted to modify the active document and be blocking in any way.