When an application registers a private class with the window procedure in a dynamic link library, it is the application's responsibility to resolve the window-procedure address before issuing this function.

A private class must not be registered with the same name as a public class in the same process.

However, if a private class is registered with the same name as one that already exists, the parameters replace the old class parameters, and the return value is TRUE. The window procedure of an existing window can be changed using WinSubclassWindow or WinSetWindowPtr. The style of an existing window can be changed with the WinSetWindowULong or WinSetWindowUShort functions. The number of bytes of storage allocated for application use cannot be changed once the window is created.

Private classes are deleted when the process that registers them terminates.