An application can create a custom public window class, but it must do so during system initialization. Only the shell can register a public window class, and it can do so only when the system starts. Registering a public window class requires a special load entry in the os2.ini file. That entry instructs the shell to load a dynamic link library whose initialization routine registers the window class. Custom public window classes must be registered using WinRegisterClass and must have the class style CS_PUBLIC. If a custom public window class registered this way has the same name as an existing public window class, the custom class replaces the original class.

If a dynamic link library replaces an existing public window class, the library can save the address of the original window procedure and use the address to subclass the original window class. The dynamic link library retrieves the original window procedure address using the WinQueryClassInfo function. The custom window procedure then passes unprocessed messages to the original window procedure instead of calling WinDefWindowProc.

When subclassing a public window class, the custom public window procedure must not make the window data size smaller than the original window data size, because all public window classes that the operating system defines use 4 extra bytes for storing a pointer to custom window data. This size is guaranteed only for public window classes defined by the operating system dynamic link libraries.