Selecting a Segment of a Polyline Visually

By arriving at New Orleans for AU2022 early I planned to explore the The Big Easy city for its unique scene and culture. But the unbearable heat (at least to me, as Canadian from far up north) forced me staying air-conditioned indoor more than I wanted. As a runner, I also planned some morning runs whenever I am in a different city, but I decided I'd better not run here to avoid unexpected heat stroke. Thus I end up having a bit more time to write some code for a question I saw in the .NET discussion forum here. While the answer to that question is fairly simple, but it still needs to be better explained with code, thus this post.

The question asked there actually involves 2 different programming tasks: identifying the selected segment of a polyline (e.g. the user should select a polyline, and the code then determine which segment of the polyline is actually clicked); and showing a visual hint to the user to indicate which segment is selected. With these 2 separate coding tasks in mind, the code then can be structured easily.

First, for identifying selected segment, I create the class PolylineSegmentSelector, which is really simple - if the user selected a polyline, based on where the picked point is, the segment's index is calculated:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Geometry;
using System;
using CadDb = Autodesk.AutoCAD.DatabaseServices;
 
namespace MiscTest
{
    public class PolylineSegmentSelector
    {
        private Document _dwg;
        private Editor _ed;
        private Database _db;
 
        public ObjectId SelectedPolyLine { private set; get; } = ObjectId.Null;
        public int SelectedSegment { private set; get; } = -1;
 
        public bool SelectSegment(Document dwg)
        {
            _dwg = dwg;
            _ed = dwg.Editor;
            _db = dwg.Database;
 
            SelectedPolyLine = ObjectId.Null;
            SelectedSegment = -1;
 
            var opt = new PromptEntityOptions(
                "\nSelect target segment of a polyline:");
            opt.SetRejectMessage("\nInvalid selection: not a polyline.");
            opt.AddAllowedClass(typeof(CadDb.Polyline), true);
            var res = _ed.GetEntity(opt);
            if (res.Status != PromptStatus.OK) return false;
 
            SelectedPolyLine = res.ObjectId;
            SelectedSegment = FindSelectedSegment(res.ObjectId, res.PickedPoint);
 
            return true;
        }
 
        private int FindSelectedSegment(ObjectId polyId, Point3d pickedPt)
        {
            int index = -1;
            using (var tran = 
                polyId.Database.TransactionManager.StartOpenCloseTransaction())
            {
                var poly = (CadDb.Polyline)tran.GetObject(polyId, OpenMode.ForRead);
                var ptOnPoly = poly.GetClosestPointTo(pickedPt, false);
                index = Convert.ToInt32(Math.Floor(poly.GetParameterAtPoint(ptOnPoly)));
                tran.Commit();
            }
 
            return index;
        }
    }
}

Then, the class PolylineSegmentHighlighter, which presents a temporary visual hint withTransient Graphics:

using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.GraphicsInterface;
using System;
using System.Collections.Generic;
using CadDb = Autodesk.AutoCAD.DatabaseServices;
 
namespace MiscTest
{
    public class PolylineSegmentHighlighter : IDisposable
    {
        private class SegmentGhost
        {
            public ObjectId PolylineId { get; set; } = ObjectId.Null;
            public int SegmentIndex { get; set; } = 0;
            public int ColorIndex { get; set; } = 0;
            public Drawable Ghost { get; set; } = null;
        }
 
        private readonly List<SegmentGhost> _ghosts = new List<SegmentGhost>();
        private TransientManager _tsManager = TransientManager.CurrentTransientManager;
 
        public void SetSegmentHighlight(ObjectId polyId, int segIndex, int colorIndex)
        {
            RemoveSegmentHighlight(polyId, segIndex);
            var newGhost = CreateHighlightGhost(polyId, segIndex, colorIndex);
            _ghosts.Add(newGhost);
            _tsManager.AddTransient(
                newGhost.Ghost, 
                TransientDrawingMode.DirectTopmost, 
                128, 
                new Autodesk.AutoCAD.Geometry.IntegerCollection());
        }
 
        public void RemoveSegmentHighlight(ObjectId polyId, int segIndex)
        {
            foreach (var ghost in _ghosts)
            {
                if (ghost.PolylineId == polyId && 
                    ghost.SegmentIndex == segIndex &&
                    ghost.Ghost!=null)
                {
                    _tsManager.EraseTransient(
                        ghost.Ghost, new Autodesk.AutoCAD.Geometry.IntegerCollection());
                    _ghosts.Remove(ghost);
                    ghost.Ghost.Dispose();
                    break;
                }
            }
        }
 
        public void Dispose()
        {
            foreach (var ghost in _ghosts)
            {
                if (ghost.Ghost != null)
                {
                    _tsManager.EraseTransient(
                        ghost.Ghost, new Autodesk.AutoCAD.Geometry.IntegerCollection());
                    ghost.Ghost.Dispose();
                }
            }
            _ghosts.Clear();
        }
 
        private SegmentGhost CreateHighlightGhost(ObjectId polyId, int segIndex, int colorIndex)
        {
            Entity ghost = null;
            using (var tran = 
                polyId.Database.TransactionManager.StartOpenCloseTransaction())
            {
                var poly = (CadDb.Polyline)tran.GetObject(polyId, OpenMode.ForRead);
                if (poly.NumberOfVertices > 2)
                {
                    DBObjectCollection ents = new DBObjectCollection();
                    poly.Explode(ents); 
 
                    for (int i = 0; i < ents.Count; i++)
                    {
                        if (i == segIndex)
                        {
                            ghost = ents[i] as Entity;
                        }
                        else
                        {
                            ents[i].Dispose();
                        }
                    }
                }
                else
                {
                    ghost = poly.Clone() as Entity;
                }
                tran.Commit();
            }
 
            if (ghost != null)
            {
                ghost.ColorIndex = colorIndex;
                return new SegmentGhost() 
                { 
                    PolylineId=polyId, 
                    SegmentIndex=segIndex, 
                    ColorIndex = colorIndex, 
                    Ghost=ghost 
                };
            }
            else
            {
                return null;
            }
        }
    }
}

Following CommandMethod demonstrates how to use these 2 classes:

[CommandMethod("SelectPolySeg")]
public static void SelectPolylineSegment()
{
    var dwg = CadApp.DocumentManager.MdiActiveDocument;
    var ed = dwg.Editor;
 
    var selectedPolySegs = new List<(ObjectId polyId, int segIndex)>();
 
    var selector = new PolylineSegmentSelector();
    using (var highlighter = new PolylineSegmentHighlighter())
    {
        int color = 1;
        while(selector.SelectSegment(dwg))
        {
            var selectedPoly = selector.SelectedPolyLine;
            var segIndex = selector.SelectedSegment;
 
            selectedPolySegs.Add((selectedPoly, segIndex));
 
            highlighter.SetSegmentHighlight(selectedPoly, segIndex, color);
            CadApp.UpdateScreen();
            color++;
        }
    }
 
    if (selectedPolySegs.Count>0)
    {
        //do whatever work required for each of the selected segments
    }
}

See the video clip below: