判断点与线段位置关系

[CommandMethod("test")]
public void Test()
{
 Database db = HostApplicationServices.WorkingDatabase;
 Editor ed = Autodesk..ApplicationServices.Application.DocumentManager.MdiActiveDocument.Editor;
 
 using (Transaction trans = db.TransactionManager.StartTransaction())
 {
  PromptPointOptions ptOpts = new PromptPointOptions("选择第一点A");
  PromptPointResult ptRes = ed.GetPoint(ptOpts);
  if (ptRes.Status != PromptStatus.OK)
  {
   throw new Autodesk.AutoCAD.Runtime.Exception();
  }
  Point3d pt1 = ptRes.Value;
  
  ptOpts = new PromptPointOptions("\n选择第二点B");
  ptRes = ed.GetPoint(ptOpts);
  if (ptRes.Status != PromptStatus.OK)
  {
   throw new Autodesk.AutoCAD.Runtime.Exception();
  }
  Point3d pt2 = ptRes.Value;
  
  //生成AB线段
  Line lineAB = new Line(pt1,pt2);
  AppendEntity(lineAB);
  trans.Commit();
  Vector3d vec1 = pt2 - pt1;
  ed.WriteMessage(vec1.ToString()+"\n");
  
  while (true)
  {
   ptOpts = new PromptPointOptions("\n选择第三点C");
   ptRes = ed.GetPoint(ptOpts);
   if (ptRes.Status != PromptStatus.OK)
   {
    throw new Autodesk.AutoCAD.Runtime.Exception();
   }
   Point3d pt3 = ptRes.Value;
   Vector3d vec2 = pt3 - pt1;
   
   //向量叉乘
   double result = vec2.X*vec1.Y-vec1.X*vec2.Y;
   
   if (result>0)
   {
    ed.WriteMessage("C点在AB线右侧");
   }else if (result<0)
   {
    ed.WriteMessage("C点在AB线左侧");
   }else if (result==0)
   {
    ed.WriteMessage("C点在AB线上");
   }
  }
 }
}
public static ObjectId AppendEntity(Entity ent)
{
 Database db = HostApplicationServices.WorkingDatabase;
 ObjectId entid;
 using (Transaction trans = db.TransactionManager.StartTransaction())
 {
  BlockTable bt = (BlockTable)trans.GetObject(db.BlockTableId,OpenMode.ForRead);
  BlockTableRecord btr = (BlockTableRecord)trans.GetObject(bt[BlockTableRecord.ModelSpace],OpenMode.ForWrite);
  entid = btr.AppendEntity(ent);
  trans.AddNewlyCreatedDBObject(ent,true);
  trans.Commit();
 }
 return entid;
}