This .NET class wraps the AcGeSplineEnt2d ObjectARX class. It represents different spline entities in 2D space.

Autodesk.AutoCAD.Geometry.Entity2d
    Autodesk.AutoCAD.Geometry.Curve2d
        Autodesk.AutoCAD.Geometry.SplineEntity2d
            Autodesk.AutoCAD.Geometry.CubicSplineCurve2d
            Autodesk.AutoCAD.Geometry.NurbCurve2d
            Autodesk.AutoCAD.Geometry.PolylineCurve2d

Public abstract Class SplineEntity2d
Inherits Curve2d

public abstract class SplineEntity2d : Curve2d;