The WM_COMMAND Message
A control can send a
WM_COMMAND message to the dialog box procedure when the user carries out an action in
the control. These messages, called notification messages, inform the procedure
of user input and permit it to carry out appropriate responses.
All predefined controls, except static controls, send notification messages
for selected user actions. For example, a push button sends the
BN_CLICKED notification message whenever the user chooses the button. In all cases, the
low-order word of the
wParam parameter contains the control identifier, the high-order word of
wParam contains the notification code, and the
lParam parameter contains the control window handle.
The dialog box procedure should monitor and process notification messages. In
particular, the procedure must process messages having the IDOK or IDCANCEL
identifiers; these messages represent a request by the user to close the dialog
box. The procedure should close the dialog box by using the
EndDialog function for modal dialog boxes and the
DestroyWindow function for modeless dialog boxes.
Windows also sends WM_COMMAND messages to the dialog box procedure if the
dialog box has a menu, such as the System menu, and the user chooses a command. In
particular, Windows sends a WM_COMMAND message with the
wParam parameter set to IDCANCEL whenever the user chooses the Close command in the
dialog box's System menu. The message is nearly identical to the notification
message sent by the Cancel button and should be processed in exactly the same
way.
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