This class represents a Horizontal constraint node in the owning Assoc2dConstraintGroup. 

It can be applied on one constrained line.

Autodesk.AutoCAD.DatabaseServices.ConstraintGroupNode
    Autodesk.AutoCAD.DatabaseServices.GeometricalConstraint
        Autodesk.AutoCAD.DatabaseServices.ParallelConstraint
            Autodesk.AutoCAD.DatabaseServices.HorizontalConstraint

Public Class HorizontalConstraint
Inherits ParallelConstraint

public class HorizontalConstraint : ParallelConstraint;