;;说明:判断点在多段线构成的多边形内侧还是外侧
;;参数:pt:点
;;参数:var:多段线构成的多边形图元名或者多边型点表
;;参数:flag:如果为T，则点在多边形上时，则判断为多边形内侧，否则则判断为外侧
;;返回:内侧为：T，外侧为：nil
;;(PtIsInPoly (getpoint) (car (entsel)) t)
;;(PtIsInPoly (getpoint) (list (getpoint) (getpoint)) t)
(defun PtIsInPoly(pt var flag / getptlst judgestart vlist)
    (defun getptlst(e) (mapcar 'cdr (vl-remove-if-not (function (lambda(x) (= 10 (car x)))) (entget e))))
    (defun judgestart(vlist / cnt diff idx len numints online p1 p1code p1x p1y p2 p2code p2x p2y vlist x xdiff xx y ydiff)
        (if (not (equal (car vlist) (last vlist)))
            (setq vlist (append vlist (list (car vlist))))
        )
        (setq X (car pt)
            Y (cadr pt)
            len (length vlist)
            cnt 0
            Idx 0
            NumInts 0
            OnLine nil
        )
        (while (and (not OnLine) (< cnt len))
            (setq
                p2 (nth cnt vlist)
                p2x (car p2)
                p2y (cadr p2)
                p2code (if (>= p2y y) 2 0)
                p2code (if (>= p2x x) (1+ p2code) p2code)
            )
            (if p1
                (setq diff (boole 6 p1code p2code)
                    ydiff (boole 1 diff 2)
                    xdiff (boole 1 diff 1)        
                    p1x (car p1)
                    p1y (cadr p1)
                )
            )
            (if (= ydiff 2)
                (progn
                    (setq xx (+ p1x (* (/ (- p2x p1x) 1. (- p2y p1y)) (- y p1y))))
                    (cond
                        ((equal xx x 0.0001) (setq online T))
                        ((> xx x) (setq NumInts (1+ NumInts)))
                    )
                )
            )
            (if (and (= xdiff 1) (= y p1y p2y)) (setq OnLine T))
            (setq p1code p2code p1 p2 cnt (1+ cnt))
        )
        (if Online flag (= (boole 1 NumInts 1) 0001))
    )
    (if (equal (type var) 'ENAME)
        (judgestart (getptlst var))
        (if (and var (listp var) (> (length var) 1)) (judgestart var) (progn (princ "\n点表输入有误！") (prin1)))
    )
)

=======================================================================================
以下原文链接
矩形只是多段线的一种解,但是矩形可以利用叉乘做到快速求解,这方面可以自行去搞搞.
这里提供一个最通用的.

测试命令:

public class JJ_test_ckk
{
    [CommandMethod("JJ_test_ckk")]
    public void TT()
    {
        Editor ed = Acap.DocumentManager.MdiActiveDocument.Editor;
        Database db = Acap.DocumentManager.MdiActiveDocument.Database;
        ed.WriteMessage(Environment.NewLine + "惊惊net测试区:");
        var ppo = new PromptPointOptions(Environment.NewLine + "测试点:")
        {
            AllowArbitraryInput = true,//任意输入
            AllowNone = true//允许回车
        };
        var ppr = ed.GetPoint(ppo);//用户点选
        if (ppr.Status != PromptStatus.OK)
        {
            return;
        }
        var peo = new PromptEntityOptions(Environment.NewLine + "点选多段线")
        {
            AllowObjectOnLockedLayer = false,
            AllowNone = false
        }; 
        var res = ed.GetEntity(peo);
        if(res.Status != PromptStatus.OK) return;   
        db.Action(tr=>
        {  
            var ent2 =res.ObjectId.ToEntity(tr);
            if (ent is Polyline pl && MathTool.IsPointInPL(ppr.Value, pl.GetPoint3ds())) 
            {
                ed.WriteMessage(Environment.NewLine + "内内内内内内内内");
            }
            else
            {
                ed.WriteMessage(Environment.NewLine + "外外外外外外外外外外外");
            }
        }); 
    }
}

id.isok

/// <summary>
/// id有效,未被删除
/// </summary>
/// <param name="id"></param>
/// <returns></returns>
public static bool IsOk(this ObjectId id)
{
    return !id.IsNull && id.IsValid && !id.IsErased && !id.IsEffectivelyErased && id.IsResident;
}

射线法:

/// <summary>
/// 判断点在闭合多段线内(线上也是内)射线法
/// </summary>
/// <param name="p">判断的点</param>
/// <param name="pts">边界点集</param>
/// <returns></returns> 
public static bool IsPointInPL(Point3d p, IEnumerable<Point3d> pts)
{
    var px = p.X;
    var py = p.Y;
    var flag = false;
    var poly = pts.ToArray();
    int l = poly.Length;
    int j = l - 1;
    for (var i = 0; i < l; i++)
    {
        var pt1x = poly[i].X;//头
        var pt1y = poly[i].Y;
        var pt2x = poly[j].X;//尾
        var pt2y = poly[j].Y;
        // 点与多边形顶点重合
        if ((pt1x.Eq(px) && pt1y.Eq(py)) || (pt2x.Eq(px) && pt2y.Eq(py)))
        {
            return true;
        }
        // 判断线段两端点是否在射线两侧
        if ((pt1y < py && pt2y >= py) || (pt1y >= py && pt2y < py))
        {
            // 线段上与射线 Y 坐标相同的点的 X 坐标
            var x = pt1x + (py - pt1y) * (pt2x - pt1x) / (pt2y - pt1y);
            // 点在多边形的边上
            if (x.Eq(px))
            {
                return true;
            }
            // 射线穿过多边形的边界
            if (x > px)
            {
                flag = !flag;
            }
        }
        j = i;
    }
    // 射线穿过多边形边界的次数为奇数时点在多边形内
    return flag;
}