Point is On Line

csharp

using System;
using Autodesk.AutoCAD.Runtime;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.EditorInput;

[assembly: CommandClass(typeof(Rivilis.CurveUtils))]

namespace Rivilis
{
  public class CurveUtils
  {
    [CommandMethod("PointOnCurve")]
    public void MyCommand() // This method can have any name
    {
      Document doc = Application.DocumentManager.MdiActiveDocument;
      Editor ed = doc.Editor;
      
      PromptEntityOptions peOpt = new PromptEntityOptions("\nSelect curve: ");
      peOpt.SetRejectMessage("\nMust be Curve!");  peOpt.AddAllowedClass(typeof(Curve), false);
      PromptEntityResult peRes = ed.GetEntity(peOpt);
      if (peRes.Status != PromptStatus.OK) return;

      PromptPointResult ppRes = ed.GetPoint("\bPick point: ");
      if (ppRes.Status != PromptStatus.OK) return;

      Point3d p = ppRes.Value.TransformBy(ed.CurrentUserCoordinateSystem);

      using (Transaction tr = doc.TransactionManager.StartTransaction()) {

        Curve crv = tr.GetObject(peRes.ObjectId, OpenMode.ForRead) as Curve;
        if (crv != null) {
          Point3d pNear = crv.GetClosestPointTo(p, false);
          if (pNear.DistanceTo(p) < Tolerance.Global.EqualPoint) {
            ed.WriteMessage("\nPoint is on Curve");
          } else {
            ed.WriteMessage("\nPoint is not on Curve");
          }
        }
      }
    }
  }
}

csharp

AutoCAD .NET: Tolerance.Global.EqualPoint & Point3d.IsEqualTo()

http://spiderinnet1.typepad.com/blog/2012/10/autocad-net-toleranceglobalequalpoint-point3disequalto.html