As the user interacts with the control, the control sends information about that interaction, in the form of a notification message, to the parent window. A notification message is a WM_COMMAND message in which:
The wParam parameter contains the control ID
The lParam parameter contains the notification code and the control handle
For example, when the user clicks a button control, that control sends a WM_COMMAND message to the window function of the parent window. The WM_COMMAND message's wParam parameter contains the button control's ID; the high-order word of lParam parameter contains the notification code BN_CLICKED, which indicates that the user has clicked that control.
Since a notification message has the same basic form as menu input, you process notification messages much as you would menu input. If you have carefully selected control IDs so that they do not conflict with menu IDs, you can process notification messages in the same switch statement you use to process menu input.