The sheet number of the document. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 23.0.0.0 (23.1.0.0) 
Syntax
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Exceptions
| Exception | Condition | 
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| Autodesk.Revit.Exceptions ArgumentException | When setting this property: sheetNumber is an empty string or contains only whitespace. -or- When setting this property: sheetNumber cannot include prohibited characters, such as "{, }, [, ], |, ;, less-than sign, greater-than sign, ?, `, ~". -or- When setting this property: Sheet number is already in use. | 
| Autodesk.Revit.Exceptions ArgumentNullException | When setting this property: A non-optional argument was null |