This .NET class wraps the AcGeCircArc3d ObjectARX class. It represents both full circles and circular arcs in 3D space.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Curve3d
        Autodesk.AutoCAD.Geometry.CircularArc3d

Public sealed Class CircularArc3d
Inherits Curve3d

public sealed class CircularArc3d : Curve3d;