An application sends the WM_MDIDESTROY message to a multiple document interface (MDI) client window to close an MDI child window.
WM_MDIDESTROY wParam = (WPARAM) (HWND) hwndChild; // handle of child to close lParam = 0; // not used; must be zero
This message always returns zero.
This message removes the title of the MDI child window from the MDI frame window and deactivates the child window. An application should use this message to close all MDI child windows.
If an MDI client window receives a message that changes the activation of its child windows and the active MDI child window is maximized, Windows restores the active child window and maximizes the newly activated child window.