{"id":44984,"date":"2015-04-29T07:00:00","date_gmt":"2015-04-29T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2015\/04\/29\/is-the-atom-returned-by-registerclassex-a-real-atom\/"},"modified":"2015-04-29T07:00:00","modified_gmt":"2015-04-29T07:00:00","slug":"is-the-atom-returned-by-registerclassex-a-real-atom","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20150429-00\/?p=44984","title":{"rendered":"Is the atom returned by RegisterClass(Ex) a "real" atom?"},"content":{"rendered":"

A customer was debugging some code that calls Register­Class<\/code> on a class that’s already been registered. In this case, it was registered by another DLL in the same process. Normally, this wouldn’t be a problem, because each DLL passes its own instance handle to Register­Class<\/code><\/a> so that there are no name collisions. However, in this case, both DLLs are passing the CS_GLOBAL­CLASS<\/code> flag, which means that collisions can occur after all.\n The customer found that the call to ERROR_CLASS_EXISTS<\/code>, which makes sense in the case of a collision in the global class table. The code then calls Find­Atom<\/code> to look up the class name, but Find­Atom<\/code> fails, saying, “No such atom.”\n Does this mean that the class atom created by Register­Class<\/code> isn’t a real<\/i> atom? How can you get the atom for a window class that somebody else has registered?\n Okay, let’s look at these questions in order.\n Is the atom a real<\/i> atom? Well, if you ask the atom, it’ll say, “I feel real to me.” But remember that there are many atom tables<\/a> in the system.<\/p>\n