This .NET class wraps the AcGeLinearEnt2d ObjectARX class. It's an abstract base class for all 2D line classes.

Autodesk.AutoCAD.Geometry.Entity2d
    Autodesk.AutoCAD.Geometry.Curve2d
        Autodesk.AutoCAD.Geometry.LinearEntity2d
            Autodesk.AutoCAD.Geometry.Line2d
            Autodesk.AutoCAD.Geometry.LineSegment2d
            Autodesk.AutoCAD.Geometry.Ray2d

Public abstract Class LinearEntity2d
Inherits Curve2d

public abstract class LinearEntity2d : Curve2d;