A flex duct in the Autodesk Revit MEP product. 
   Namespace:   Autodesk.Revit.DB.Mechanical  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
Inheritance Hierarchy
  System Object  
Autodesk.Revit.DB Element
Autodesk.Revit.DB HostObject
Autodesk.Revit.DB MEPCurve
Autodesk.Revit.DB.Mechanical FlexDuct
 
 Autodesk.Revit.DB Element
Autodesk.Revit.DB HostObject
Autodesk.Revit.DB MEPCurve
Autodesk.Revit.DB.Mechanical FlexDuct