This .Net class wraps AcDbSolid ObjectARX class. 

The Solid class is the representation for the SOLID entity within AutoCAD. The SOLID entity is a 2D entity, not the 3D solids that are represented by the 3dSolid and Body classes.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.Solid

Public Class Solid
Inherits Entity

public class Solid : Entity;