# What happens if you change a window class's GCL\_CBWNDEXTRA?

DevFeed: [What happens if you change a window class's GCL\_CBWNDEXTRA?](<https://devfeed.tech/articles/what-happens-if-you-change-a-window-class-s-gcl-cb-wnd-extra-21756.md>)

Original publisher: [Read original article](<https://devblogs.microsoft.com/oldnewthing/20260904-00/?p=112675>)

Author: Raymond Chen

Published: 2026-09-04T14:00:00Z

Content type: article

Language: en

Sources: [Raymond Chen](<https://devfeed.tech/sources/raymond-chen.md>)

Topics: [Windows](<https://devfeed.tech/topics/windows.md>), [Code](<https://devfeed.tech/topics/code.md>)

Tags: [code](<https://devfeed.tech/tags/code.md>), [compatibility](<https://devfeed.tech/tags/compatibility.md>), [old-new-thing](<https://devfeed.tech/tags/old-new-thing.md>), [post](<https://devfeed.tech/tags/post.md>), [windows](<https://devfeed.tech/tags/windows.md>)

## AI overview

The article explains that changing a window class's extra-byte size affects only windows created afterward. Existing windows retain the size assigned at creation, and accessing bytes that were not available then can produce an out-of-range result.

## Source excerpt

You're changing the value for future windows. The post What happens if you change a window class's <CODE>GCL_<WBR>CBWNDEXTRA</CODE>? appeared first on The Old New Thing.