The dimension value as a user visible string. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
Remarks
 This property always return    a null reference (  Nothing  in Visual Basic)  if this is a spot dimension, or it is a linear dimension with more than one segments.