这是G版的代码...但你要的似乎首尾相连且共线才连接...

(defun c:tt (/ gxl-Sel-ReDrawSel gxl-Sel-SSsub gxl-Sel-SSJoin gxl-sel-SSgetLineatPoint getline)

(defun gxl-Sel-ReDrawSel (Sel mode / i)

(repeat (setq i (sslength Sel))

(redraw (ssname Sel (setq i (1- i))) mode)

)

)

(defun gxl-Sel-SSsub(ss1 ss2 / ss i)

(cond

((and ss1 ss2)

(repeat (setq i (sslength ss2))

(ssdel (ssname ss2 (setq i (1- i))) ss1)

)

)

((and ss1 (not ss2)) ss1)

(T (setq ss1 nil))

)

ss1

)

(defun gxl-Sel-SSJoin ( ss1 ss2 / ss i)

(setq ss (ssadd))

(if ss1 (if (= (type ss1) 'ENAME) (ssadd ss1 ss)))

(if ss2 (if (= (type ss2) 'ENAME) (ssadd ss2 ss)))

(if (and ss1 ss2)

(progn

(repeat (setq i (sslength ss2))

(ssadd (ssname ss2 (setq i (1- i))) ss)

)

(repeat (setq i (sslength ss1))

(ssadd (ssname ss1 (setq i (1- i))) ss)

)

))

(if (and ss1 (not ss2)) (setq ss ss1))

(if (and ss2 (not ss1)) (setq ss ss2))

(if (> (sslength ss) 0) ss nil)

)

(defun gxl-sel-SSgetLineatPoint        (pt jd / p1 p2 ss)

(setq        p1 (mapcar '- pt (list jd jd))

p2 (mapcar '+ pt (list jd jd))

ss (ssget "c" p1 p2 '((0 . "line")))

)

(if ss (GXL-SEL-REDRAWSEL ss 3))

ss

)

(defun getline (pt jd / s s1 i p1 p2)

(setq s (gxl-sel-SSgetLineatPoint pt jd))

(if s

(progn

(setq s1 (GXL-SEL-SSSUB s ssrtl)

ssrtl (GXL-SEL-SSJOIN ssrtl s1)

)

(if s1

(repeat (setq i (sslength s1))

(setq e  (ssname s1 (setq i (1- i)))

p1 (cdr (assoc 10 (entget e)))

p2 (cdr (assoc 11 (entget e)))

)

(getline p2 jd)

(getline p1 jd)

)

)

)

)

)

(setq e (car (entsel "\n选择直线:")))

(initget 5 "  ")

(setq jd (getreal "输入容差精度：<0.001>"))

(if (= jd "") (setq jd 0.001))

(setq pt1 (cdr (assoc 10 (entget e))))

(setq pt2 (cdr (assoc 11 (entget e))))

(setq ssrtl (ssadd e))

(getline pt1 jd)

(getline pt2 jd)

(sssetfirst nil ssrtl)

)