Lesson 5.17 – Basic Error Handling in AutoLISP

🚨 Lesson 5-17 – Basic Error Handling in AutoLISP

Learn how to use the *error* function to prevent crashes and restore your drawing environment after unexpected issues.

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📚 What You’ll Learn

By the end of this lesson, you’ll be able to:

• Implement custom error handlers using the *error* function
• Restore environment settings like OSNAP or layer states
• Safely exit routines when something goes wrong
• Write more professional, resilient AutoLISP tools

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🧠 Why It Matters

Even a small bug can disrupt your workflow or leave settings changed in your drawing session. With proper error handling, you can trap problems, display helpful messages, and return AutoCAD to a clean state automatically.

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🛠️ Tools You’ll Use

Tool / Function	Purpose
*error*	Custom global error function
(command ...)	Reset settings during cleanup
setq / setvar	Store and restore AutoCAD system variables
(princ)	Suppress command line clutter

🧭 Lesson Structure

1️⃣ The Anatomy of an AutoLISP Error

When something goes wrong—such as a canceled command or an invalid selection—AutoCAD may:

• Display a system error message
• Exit your function prematurely
• Leave system variables (like OSNAP) in a changed state

This is why cleanup is important.

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2️⃣ Introducing the *error* Function

Here’s a basic example:

(defun my-error (msg)
  (prompt (strcat "\\n⚠ Error: " msg))
  (setvar "OSMODE" 0) ; Restore OSNAP to default
  (setq *error* nil)  ; Reset handler
  (princ)
)

Set this function as the error handler at the start of your routine:

(setq olderror *error*
      *error* my-error)

3️⃣ Complete Pattern with Error Handling and Cleanup

(defun c:safeoffset ( / *error* oldsnap ent dist )

  ;; Save OSNAP setting
  (setq oldsnap (getvar "OSMODE"))

  ;; Define error handler
  (defun my-error (msg)
    (prompt (strcat "\\n⚠ Error: " msg))
    (setvar "OSMODE" oldsnap)
    (setq *error* olderror)
    (princ)
  )

  ;; Set the error handler
  (setq olderror *error* *error* my-error)

  ;; Routine logic
  (setq ent (car (entsel "\\nSelect object to offset: ")))
  (setq dist (getdist "\\nEnter offset distance: "))
  (command "_.offset" dist ent pause "")

  ;; Restore environment
  (setvar "OSMODE" oldsnap)
  (setq *error* olderror)
  (princ)
)

4️⃣ Error Types You Can Catch

Example Error Message	What It Means
Function cancelled	User hit ESC or clicked cancel
bad argument type	Function expected a number but got nil
no function definition	Misspelled function or uninitialized call

Use (alert msg) instead of (prompt) if you want to show a dialog box.

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✅ Lesson Checklist

Task	Completed
Created a routine with a custom *error* handler	☐
Saved and restored at least one system variable	☐
Tested user cancellation or bad input	☐
Used (princ) to suppress command output	☐

📌 Quick Tips

Tip	Why It Helps
Always restore system variables	Prevents unexpected drawing behavior
Reset *error* to its previous value	Avoids recursive or unintended trapping
Use princ often	Makes command output clean and quiet
Name your error handler clearly	Easier to reuse and debug

🧩 Real-World Applications

Scenario	Why Error Handling Helps
User cancels command	Restores previous settings and shows friendly output
Invalid selection	Prevents crash and prompts user to retry
OSNAP or layer settings	Maintains consistent drawing environment

📁 Files and Resources

File / Resource	Description
safeoffset_example.lsp	Complete routine with full error handling
error_template.lsp	Blank template to copy and modify
error_handling_checklist.docx	Printable checklist to guide proper cleanup

📖 Review Table

Function	Purpose	Example
*error*	Sets global error function	(setq *error* my-error)
setvar	Restores AutoCAD system settings	(setvar "OSMODE" oldsnap)
entsel, getdist	Gets user input	(getdist "\\nDistance: ")
command	Calls AutoCAD commands from LISP	(command "OFFSET" dist ent pause "")