A compatibility note on the abuse of Windows window class extra bytes

# A compatibility note on the abuse of Windows window class extra bytes - The Old New Thing Source: [https://devblogs.microsoft.com/oldnewthing/20260630-00?p=112488](https://devblogs.microsoft.com/oldnewthing/20260630-00?p=112488) June 30th, 2026 1 reaction  During my discussion of[the evolution of system\-windows window and class extra bytes](https://devblogs.microsoft.com/oldnewthing/20260629-00/?p=112484), I noted that even though IDs are typically small integers, people liked to stash pointers there, so we had to expand the ID field to a pointer\-sized integer\. One thing I’ve learned is that anywhere it’s possible to hide a pointer, people will hide a pointer there\. This is true even for small integers\. As I was digging up the history of the extra bytes, I saw a special note in the 16\-bit code for`Set­Class­Word`: It says that there’s an app that expects to be able to modify the value of`GWW\_CB­CLS­EXTRA`\. Now, modifying this value has no practical effect because the memory for the class was allocated when you called`Register­Class`\. You can’t go back in time and change the allocation size\. But one program realized that it could use this value as a place to store some private data, so they did\. Sure, that’s not the purpose of the`GWW\_CB­CLS­EXTRA`, but that never stopped them\. For compatibility, Windows lets 16\-bit programs modify`GWW\_CB­CLS­EXTRA`\. But at least it blocks it for 32\-bit and 64\-bit programs\. One loophole closed\. Countless more to go\. ### Category ### Topics ## Author  Raymond has been involved in the evolution of Windows for more than 30 years\. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie\-jeebies\. The Web site spawned a book, coincidentally also titled The Old New Thing \(Addison Wesley 2007\)\. He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information\. ## Read next ## Stay informed Get notified when new posts are published\. Follow this blog - [https://twitter.com/ChenCravat](https://twitter.com/ChenCravat) - [](https://www.youtube.com/playlist?list=PLlrxD0HtieHge3_8Dm48C0Ns61I6bHThc) - [https://github.com/oldnewthing](https://github.com/oldnewthing) - [https://devblogs.microsoft.com/oldnewthing/feed/](https://devblogs.microsoft.com/oldnewthing/feed/)