VC CreateWindow cannot find the window class

VC CreateWindow cannot find the window class, there is a hint because there is no call registerclass Registration window class, but I have used registerclassex registered!
Function: MyRegisterClass
Role: Register window class

ATOM MyRegisterClass (hinstance hinstance) {wndclassex wndclass;Wndclass. cbsize= sizeof (WNDCLASS);Wndclass. Style= Cs_hredraw | Cs_vredraw;Wndclass. Lpfnwndproc= WndProc;Wndclass. Cbclsextra=0;Wndclass. Cbwndextra=0;Wndclass. HInstance= HInstance;Wndclass. Hicon= LoadIcon (NULL, idi_application);Wndclass. Hcursor= LoadCursor (NULL, Idc_arrow);Wndclass. Hbrbackground= (Hbrush) getstockobject (White_brush);Wndclass. Lpszmenuname= NULL;Wndclass. lpszClassName= Szappname;return RegisterClassEx (&wndclass);}

Later, instead of using registerclass, the problem was solved:
Function: MyRegisterClass
Role: Register window class

ATOM MyRegisterClass (hinstance hinstance) {wndclass wndclass;Wndclass. cbsize= sizeof (WNDCLASS);Wndclass. Style= Cs_hredraw | Cs_vredraw;Wndclass. Lpfnwndproc= WndProc;Wndclass. Cbclsextra=0;Wndclass. Cbwndextra=0;Wndclass. HInstance= HInstance;Wndclass. Hicon= LoadIcon (NULL, idi_application);Wndclass. Hcursor= LoadCursor (NULL, Idc_arrow);Wndclass. Hbrbackground= (Hbrush) getstockobject (White_brush);Wndclass. Lpszmenuname= NULL;Wndclass. lpszClassName= Szappname;Return RegisterClass (&wndclass);}

VC CreateWindow cannot find the window class