com-api-pitfalls

COM API Pitfalls for LLM Agents

This page documents hard-won lessons from debugging the SolidWorks COM bridge. If you are an AI coding assistant (Claude, GPT-4, Gemini, Copilot, etc.) working on this codebase, read this page before touching any COM-related code. Every entry below caused a real runtime failure that required hours to diagnose.

!!! tip "For LLM agents" The patterns here are not obvious from the SolidWorks API docs alone. They are the delta between "what the documentation says" and "what actually works" under pywin32 late-binding on SW 2025/2026.

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1. SelectByID2 — Callout must be VT_DISPATCH null, not None

Symptom: (-2147352571, 'Type mismatch.', None, 8) when calling SelectByID2.

Root cause: The Callout parameter (8th argument, type VT_DISPATCH) expects a COM null pointer, not a Python None. Python None marshals as VT_NULL which SolidWorks rejects with DISP_E_TYPEMISMATCH.

Fix:

import pythoncom
import win32com.client as _win32com

null_callout = _win32com.VARIANT(pythoncom.VT_DISPATCH, None)
model.Extension.SelectByID2(
    "", "EDGE", x, y, z,
    append, mark,
    null_callout,   # <- NOT plain None
    0,
)

Applies to: Every call to SelectByID2 or SelectByID that doesn't need a real callout object. The same issue occurs for FACE, EDGE, VERTEX, and other entity types.

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2. InsertFeatureChamfer lives on IFeatureManager, not IModelDocExtension

Symptom: <unknown>.InsertFeatureChamfer error when calling through model.Extension.

Root cause: InsertFeatureChamfer (DISPID 83) is a method of IFeatureManager, not IModelDocExtension. Routing it through model.Extension causes DISP_E_MEMBERNOTFOUND.

Fix:

import math

fm = model.FeatureManager          # IFeatureManager (property access, no parens)
feature = fm.InsertFeatureChamfer(
    1,                             # Options
    1,                             # ChamferType = swChamferEqualDistance
    distance_m,                    # Width in metres
    math.pi / 4,                   # Angle (45 degrees)
    0.0,                           # OtherDist (unused for equal-distance)
    0.0, 0.0, 0.0,                 # VertexChamDist1, 2, 3 (unused)
)