Implement this method to handle the drop event for your data. 
   Namespace:   Autodesk.Revit.UI  
  Assembly:   RevitAPIUI  (in RevitAPIUI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2013 
Syntax
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Parameters
- document
 -  Type:  Autodesk.Revit.UI UIDocument  
The document on which the data was dropped. 
- data
 -  Type:  System Object  
The data.