This .Net class wraps AcGeSplineEnt3d ObjectARX class. It represents different spline entities in 3D space.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Curve3d
        Autodesk.AutoCAD.Geometry.SplineEntity3d
            Autodesk.AutoCAD.Geometry.CubicSplineCurve3d
            Autodesk.AutoCAD.Geometry.NurbCurve3d
            Autodesk.AutoCAD.Geometry.PolylineCurve3d

Public abstract Class SplineEntity3d
Inherits Curve3d

public abstract class SplineEntity3d : Curve3d;