This .NET class wraps the AcDb2dPolyline ObjectARX class. 

The Polyline2d class represents the 2D polyline entity within AutoCAD.

Autodesk.AutoCAD.DatabaseServices.DBObject
    Autodesk.AutoCAD.DatabaseServices.Entity
        Autodesk.AutoCAD.DatabaseServices.Curve
            Autodesk.AutoCAD.DatabaseServices.Polyline2d

Public Class Polyline2d
Inherits Curve
Inherits IEnumerable

public class Polyline2d : 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, Vertex2d