The fill pattern Id. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 19.0.0.0 (19.0.0.405) 
  Since:  2013 
Syntax
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Exceptions
| Exception | Condition | 
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| Autodesk.Revit.Exceptions ArgumentException | When setting this property: The element id must represent a valid FillPatternElement. -or- When setting this property: The FillPattern target must be a drafting pattern. | 
| Autodesk.Revit.Exceptions ArgumentNullException | When setting this property: A non-optional argument was NULL |