This wraps the ObjectARX AcDbAssocExtendSurfaceActionBody class.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.AssocActionBody
        Autodesk.AutoCAD.DatabaseServices.AssocParamBasedActionBody
            Autodesk.AutoCAD.DatabaseServices.AssocSurfaceActionBody
                Autodesk.AutoCAD.DatabaseServices.AssocPathBasedSurfaceActionBody
                    Autodesk.AutoCAD.DatabaseServices.AssocExtendSurfaceActionBody

Public Class AssocExtendSurfaceActionBody
Inherits AssocPathBasedSurfaceActionBody

public class AssocExtendSurfaceActionBody : AssocPathBasedSurfaceActionBody;