This .NET class wraps the AcGePolyline3d ObjectARX class. It's represents a piecewise linear spline entity in 3D space.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Curve3d
        Autodesk.AutoCAD.Geometry.SplineEntity3d
            Autodesk.AutoCAD.Geometry.PolylineCurve3d
                Autodesk.AutoCAD.Geometry.AugmentedPolylineCurve3d

Public Class PolylineCurve3d
Inherits SplineEntity3d

public class PolylineCurve3d : SplineEntity3d;