Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc.) —

Mathias Jonathan — Tue, 21 Jul 2026 10:19:58 GMT

Hi,

I'm trying to programmatically change GDL library-part parameters (e.g. bOverridePostSurface / matPost) on Railing sub-elements — Posts, Rails, Panels, etc. — identified by their own GUID (obtained via ACAPI_Element_GetMemo_ExternalHierarchical / the equivalent subelement enumeration).

Reading works fine: ACAPI_Element_GetMemo(subElemGuid, &memo, APIMemoMask_AddPars) correctly returns the full AddPar list for a sub-element (e.g. 111 parameters for a Post, including bOverridePostSurface/matPost).

Writing does not work, and I've been unable to find a supported path. What I've tried, each rejected cleanly (no crash, but no success):

ACAPI_LibraryPart_OpenParameters (session-based) on the sub-element — fails outright; can't get a session open for this element type.
ACAPI_Element_Change(&element, &mask, &memo, APIMemoMask_AddPars, true) directly on the sub-element's own guid — APIERR_BADPARS. (Tried both an empty mask and ACAPI_ELEMENT_MASK_SETFULL.)
ACAPI_Element_ChangeParameters(elemGuids, &element, &memo, &mask) — APIERR_BADID.
ACAPI_Element_ChangeExt using API_SubElement with subType = APISubElement_RailingPost, called on the owning top-level Railing element — APIERR_BADID, tried both with the sub-element's real GUID set and with the fully-populated API_Element copied into subElem, indices left at subIndex = subSubIndex = 0. Same rejection both times.

I noticed API_SubElementType defines APISubElement_RailingPost, RailingRail, etc., and API_SubElement::subIndex/subSubIndex's field docs specifically mention Railing — but API_SubElement's own class-level description only lists Window/Door/Section/Elevation/InteriorElevation/ChangeMarker/CurtainWall as the element types supported by ACAPI_Element_GetDefaultsExt/ChangeDefaultsExt/CreateExt/ChangeExt. Railing isn't mentioned there, which would explain the consistent APIERR_BADID.

I also confirmed there's no structural path through the parent: none of API_RailingPostType/API_RailingNodeType/API_RailingSegmentType/etc. carry a params/AddPar handle anywhere in the struct tree, so there's nothing to modify even via the parent Railing's own memo.

My question: is writing AddPar values to a Railing sub-element currently supported by the public API at all, through any function? If ACAPI_Element_ChangeExt is meant to support it, what are the correct semantics for subIndex/subSubIndex for APISubElement_RailingPost specifically (segment index? node index? position within a pattern?), and does subElem's GUID matter at all, or is addressing purely index-based?

Thanks!

Operating system used: Windows

Re: Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc.

Tamas Polyak — Mon, 03 Aug 2026 06:41:11 GMT

Hi Mathias,

I've opened a support issue about your question.

Regards,

Tamás

Re: Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc.

Tamas Polyak — Wed, 19 Aug 2026 16:07:05 GMT

Hi Mathias,

I've got an answer from the developer, please find it below.

Regards,

Tamás

Does this workflow work, or? If not, which point fails?

Obtain the editable Post/Rail/Panel sub-element GUID from the owner railing memo.
Call ACAPI_Element_Get using that sub-element GUID.
Call ACAPI_Element_GetMemo(guid, &memo, APIMemoMask_AddPars).
Modify memo.params
1. bOverridePostSurface: Boolean stored in value.real
2. matPost: Material stored in value.real
Call

ACAPI_Element_Change ( &element, nullptr, &memo, APIMemoMask_AddPars, true );
Check the result and dispose the memo.

topic Re: Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc. in Archicad C++ API

Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc.) —

Re: Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc.

Re: Writing GDL AddPar values (e.g. material override) to Railing sub-elements (Post/Rail/Panel/etc.