How to access window class extra memory without a window handle?

I need to register a window class, and right after that set some data in it's extra memory section (the one cbClsExtra refers to) before any window of the class is being created. The problem is that SetClassLong expects a HWND in orde开发者_JAVA技巧r to identify the class, instead of the class's name or atom.

What's the way to acheive this?

Why can't you just create a dummy window of that class, set the data and destroy the window?