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ERROR_PRIVATE_DIALOG_INDEX
The index belongs to private dialog storage
Dialog windows use reserved extra bytes and predefined DWLP_* offsets for dialog-manager state such as the dialog procedure and message result. Those offsets are not ordinary application-defined window storage.
The problem often comes from generic subclassing or framework code that treats every HWND as if its extra bytes were registered by the application. Writing through a guessed offset can collide with dialog bookkeeping even if the same helper works for a custom class.
What to check
- Determine whether the target window is a dialog or dialog-derived window.
- Use
DWLP_MSGRESULT,DWLP_DLGPROC, and documented dialog APIs only for their defined purposes. - Store application state with a documented pointer field, property, subclass mechanism, or deliberately reserved class extra bytes.
- Audit 32/64-bit code for use of the pointer-width window-data APIs.
This is narrower than ERROR_INVALID_INDEX: the index may be recognizable, but it refers to dialog-private storage that the caller must not treat as general-purpose data.
References
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