The Id of the internal load element associated with a boundary conditions. 
   Namespace:   Autodesk.Revit.DB.Structure  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 16.0.0.0 (16.0.0.0) 
  Since:  2016 
Syntax
Remarks
 ElementId may be set if the internal load exists and it's type fit the BoundaryConditions type only (Point, Line and Area). 
 Exceptions
| Exception | Condition | 
|---|---|
| Autodesk.Revit.Exceptions ArgumentException | When setting this property: elementId is not a valid Element identifier. -or- When setting this property: Throws when the ElementId does not refer to the internal load with appropriate type. | 
| Autodesk.Revit.Exceptions ArgumentNullException | When setting this property: A non-optional argument was NULL |