This .NET class wraps the AcGeCurve2d ObjectARX class. It's an abstract base class for all 2D curves. Any class that is derived from this class represents a 2D curve.

Autodesk.AutoCAD.Geometry.Entity2d
    Autodesk.AutoCAD.Geometry.Curve2d
        Autodesk.AutoCAD.Geometry.CircularArc2d
        Autodesk.AutoCAD.Geometry.CompositeCurve2d
        Autodesk.AutoCAD.Geometry.EllipticalArc2d
        Autodesk.AutoCAD.Geometry.ExternalCurve2d
        Autodesk.AutoCAD.Geometry.LinearEntity2d
        Autodesk.AutoCAD.Geometry.OffsetCurve2d
        Autodesk.AutoCAD.Geometry.SplineEntity2d

Public abstract Class Curve2d
Inherits Entity2d

public abstract class Curve2d : Entity2d;