Windows error 1412 (ERROR_CLASS_HAS_WINDOWS): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: the class cannot be unregistered while windows created from it still exist.

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ERROR_CLASS_DOES_NOT_EXIST	ERROR_INVALID_INDEX

ERROR_CLASS_HAS_WINDOWS

Live windows still use the class

UnregisterClass requires every window created with the specified class to be destroyed first. The window class is a template shared by its instances; removing the template while instances remain is prohibited.

Hidden windows, message-only windows, modeless dialogs, and windows created on secondary UI threads are common reasons an application believes cleanup is complete when a class still has instances. Destroying only the visible main window may therefore be insufficient.

What to check

Enumerate or otherwise track every window instance created from the class.
Shut down secondary GUI threads and message-only helper windows before class unregistration.
Destroy modeless windows explicitly and wait for their owner threads to finish.
Call UnregisterClass only after the instance count has reached zero.

This is different from ERROR_CLASS_ALREADY_EXISTS: the registration is valid, but teardown order prevents removing it.

References

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