AutoCAD C polyline self intersection detection

Mainly through Self and self “IntersectWith” operation , The end point and intersection point will be returned .

Source of ideas ：How do I know whether the polyline overlap itself? - Autodesk Community - AutoCAD

IntersectWith Result chart ：

 

#region  Polyline self intersection detection 

        /// <summary>
        ///  Polyline self intersection detection 
        /// </summary>
        /// <param name="pPolyline"> Polyline to be tested </param>
        /// <param name="intersectPoint3Ds"> Returns the set of self intersection points </param>
        /// <param name="error"></param>
        /// <returns></returns>
        public static bool SelfIntersectDetect(Polyline pPolyline, out List<Point3d> intersectPoint3Ds, out Exception error)
        {
            error = null;
            intersectPoint3Ds = new List<Point3d>();
            try
            {
                //  The result of self intersecting with itself （ Contains vertices and intersections ）
                var intersectWithResult = new Point3dCollection();
                pPolyline.IntersectWith(pPolyline, Intersect.OnBothOperands, intersectWithResult, IntPtr.Zero, IntPtr.Zero);

                //  Store vertices 
                HashSet<Point3d> vertices = new HashSet<Point3d>();
                int count = pPolyline.NumberOfVertices;
                for (int j = 0; j < count; j++)
                {
                    Autodesk.AutoCAD.Geometry.Point2d vertexd = pPolyline.GetPoint2dAt(j);
                    vertices.Add(new Point3d(vertexd.X, vertexd.Y, 0.0));
                }

                count = intersectWithResult.Count;
                for (int j = 0; j < intersectWithResult.Count; j++)
                {
                    Point3d tempPoint3D = intersectWithResult[j];
                    //  Weeding vertices , Get the real intersection 
                    if (!vertices.Contains(tempPoint3D))
                    {
                        intersectPoint3Ds.Add(tempPoint3D);
                    }
                }


                return true;
            }
            catch (Exception ex)
            {
                error = ex;
                return false;
            }
        }

        #endregion