This .NET class wraps the AcGeCircArc2d ObjectARX class. It represents both full circles and circular arcs in 2D space.

Autodesk.AutoCAD.Geometry.Entity2d
    Autodesk.AutoCAD.Geometry.Curve2d
        Autodesk.AutoCAD.Geometry.CircularArc2d

Public sealed Class CircularArc2d
Inherits Curve2d

public sealed class CircularArc2d : Curve2d;