A custom common dialog box is a common dialog box that has been altered to suit a particular Windows application. The customization may be complex and include the hiding of original controls, the addition of new controls, or a change in the size of the original dialog box; or it may be simple, such as the alteration of a single existing control.
Developers who need to customize a common dialog box must provide a special hook function and, in most cases, a custom dialog box template. Customizations of this kind require a significant amount of additional code—displaying a customized common dialog box is not as simple as initializing the members of a structure and calling a single function.
Applications that subclass controls in any of the common dialog boxes must do so while processing the WM_INITDIALOG message in the application's hook function. This allows the application to receive the control-specific messages first, because it will have subclassed the control after the common dialog box has installed its subclassing procedures. (The previous hook function should be called for all messages that are not handled by the application's subclass function, as is standard for subclassing.)
An application cannot subclass a control by defining a local class to override a specific control type. The reason is that the data segment would not be correctly initialized when the class was called—the data segment would be the common dialog box's data segment, not the application's data segment.