This .NET class wraps the AcGeCompositeCurve3d ObjectARX class. 

Represents a composite curve, which is a single curve that consists of any number of component curves that are connected end-to-end. For instance, a composite curve could consist of four straight line segments that form a square. Each of the component curves must be bounded curves, which means that infinite lines and rays may not be used as component curves of a composite. The component curves are not required to all lie in the same plane. A composite curve object stores a list of pointers to the component curves. 

The start parameter of a composite is 0.0. The parameter value at the endpoint of the first component curve is the approximate length of the first curve (call this value l1). The parameter value at the endpoint of the second curve is l1 plus the approximate length of the second curve, etc.

Autodesk.AutoCAD.Geometry.Entity3d
    Autodesk.AutoCAD.Geometry.Curve3d
        Autodesk.AutoCAD.Geometry.CompositeCurve3d

Public sealed Class CompositeCurve3d
Inherits Curve3d

public sealed class CompositeCurve3d : Curve3d;