The example in Creating a Combo-box Toolbar processes the WM_CREATE message to create two combo box controls as child windows. It then subclasses the edit controls (selection fields) in each combo box because they receive the character input for simple and drop-down combo box. The application gets the handle to each edit control by using the ChildWindowFromPoint function.
To subclass the edit controls, the application calls the SetWindowLong function, replacing the address of the class window procedure with the address of the application-defined SubClassProc function. The address of the original window procedure is saved in the global variable lpfnEditWndProc.
SubClassProc intercepts tab, esc, and enter keys and notifies the toolbar window by sending application-defined messages (WM_TAB, WM_ESC, and WM_ENTER). SubClassProc uses the CallWindowProc function to pass most messages to the original window procedure, lpfnEditWndProc.