How the System Locates a Window Class

The system maintains a list of structures for each of the three types of window classes. When an application calls the CreateWindow or CreateWindowEx function to create a window with a specified class, the system uses the following procedure to locate the class:

1. Search the list of application local classes for a class with the specified name whose instance handle matches the module's instance handle. (Several modules can use the same name to register local classes in the same process.)
2. If the name is not in the application local class list, search the list of application global classes.
3. If the name is not in the application global class list, search the list of system classes.

All windows created by the application use this procedure, including windows created by the system on the application's behalf, such as dialog boxes. It is possible to override system classes without affecting other applications. That is, an application can register an application local class having the same name as a system class. This replaces the system class in the context of the application but does not prevent other applications from using the system class.