This .NET class wraps the AcDb3dPolyline ObjectARX class. 

The Polyline3d class represents the 3D polyline entity within AutoCAD.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.Curve
            Autodesk.AutoCAD.DatabaseServices.Polyline3d

Public Class Polyline3d
Inherits Curve
Inherits IEnumerable

public class Polyline3d : Curve, IEnumerable;

The controlling application must explicitly delete the subentities of any complex entity that is not database resident. Otherwise, a small memory leak will result. 

Do not derive from this class. Attempting to do so will cause your application to halt AutoCAD. 

DBObject, Entity