WMNCDESTROY

WMNCDESTROY, properly named WM_NCDESTROY in the Win32 API, is a Windows message sent to a window procedure when the non-client area of a window is being destroyed. The term WMNCDESTROY is a common shorthand found in documentation and code, but the official constant is WM_NCDESTROY.

This message concerns the non-client area, which includes elements outside the window’s client drawing area such as borders, caption bar, and menus. WM_NCDESTROY is delivered as part of the window destruction process, before the WM_DESTROY message. It gives the window a final chance to release resources tied to non-client rendering, such as handles to non-client drawing resources or any cleanup that depends on the non-client frame.

Handling guidance: a window procedure may process WM_NCDESTROY to perform necessary cleanup of non-client resources and then return control to the system. If the message is not handled, it should be passed to DefWindowProc for default processing. After WM_NCDESTROY is processed, the system will typically proceed to WM_DESTROY and then destroy the window handle.

In summary, WM_NCDESTROY signals the impending destruction of a window’s non-client frame, enabling appropriate cleanup prior to the overall window destruction. It is part of the standard window lifecycle in Win32 programming, occurring before WM_DESTROY.