autolisp-development

AutoLISP Development

Guide for writing correct AutoLISP/Visual LISP code using the oarx-docs MCP server.

Critical Rule

ALWAYS look up AutoLISP functions before using them. AutoLISP has hundreds of functions with specific argument patterns. Use the documentation.

lookup_class("vl-cmdf", guide="AutoCAD-AutoLISP-Reference")
search_docs("selection set filters", guide="AutoCAD-AutoLISP")

Guide Selection

• Function syntax → AutoCAD-AutoLISP-Reference
• Concepts and patterns → AutoCAD-AutoLISP
• Tutorials → AutoCAD-AutoLISP-Tutorials
• ActiveX/COM objects → AutoCAD-ActiveX-Reference
• DCL dialogs → search AutoCAD-AutoLISP for "DCL"

Required Patterns

Error Handling

Always define an error handler with cleanup:

(defun c:MYCMD (/ *error* old-osmode)
  (defun *error* (msg)
    (if old-osmode (setvar "OSMODE" old-osmode))
    (if (/= msg "Function cancelled")
      (princ (strcat "\nError: " msg))
    )
    (princ)
  )
  (setq old-osmode (getvar "OSMODE"))
  (setvar "OSMODE" 0)
  ;; ... main logic ...
  (setvar "OSMODE" old-osmode)
  (princ)
)

Command Execution

;; PREFERRED: vl-cmdf (returns T/nil, no pause)
(vl-cmdf "_.LINE" pt1 pt2 "")

;; ALTERNATIVE: command (legacy, may pause for input)
(command "_.LINE" pt1 pt2 "")

Look up the difference:

lookup_class("vl-cmdf", guide="AutoCAD-AutoLISP-Reference")
lookup_class("command", guide="AutoCAD-AutoLISP-Reference")

Selection Sets

;; Select all lines on layer "WALLS"
(setq ss (ssget "X" '((0 . "LINE") (8 . "WALLS"))))
(if ss
  (repeat (setq i (sslength ss))
    (setq i (1- i))
    (setq ent (ssname ss i))
    (setq data (entget ent))
    ;; process entity...
  )
)

ActiveX/COM Integration

;; Get ActiveX object from entity
(setq obj (vlax-ename->vla-object ent))
(vla-get-StartPoint obj)
(vla-put-Color obj acRed)

Look up available properties/methods:

lookup_class("vla-get-StartPoint", guide="AutoCAD-ActiveX-Reference")