This .Net class wraps the ObjectARX class AcDbSurface.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.Surface
            Autodesk.AutoCAD.DatabaseServices.LoftedSurface
            Autodesk.AutoCAD.DatabaseServices.PlaneSurface
            Autodesk.AutoCAD.DatabaseServices.SweptSurface

Public Class Surface
Inherits Entity

public class Surface : Entity;