Создание простейшего окна средствами WinApi

#include <windows.h>
 
//объявление функций
BOOL RegClass(WNDPROC,LPCSTR,UINT);
LRESULT CALLBACK WndProc(HWND,UINT,WPARAM,LPARAM);
 
//описание глобальных переменных
HINSTANCE hInst;
char szClassName[]="WindowAppClass";
 
//описание главной функции
int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpszCmdLine, int nCmdShow)
{
    MSG msg;
    HWND hwnd;
    hInst=hInstance;
    if(!RegClass(WndProc,szClassName,COLOR_WINDOW))
        return FALSE;
    hwnd = CreateWindow(szClassName, "Elemental application", WS_OVERLAPPEDWINDOW|WS_VISIBLE,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,CW_USEDEFAULT,0,0,hInstance,NULL);
    if(!hwnd) return FALSE;
    while(GetMessage(&msg,0,0,0)) DispatchMessage(&msg);
    return msg.wParam;
}
 
//описание функции регистрации классов
BOOL RegClass(WNDPROC Proc, LPCTSTR szName, UINT brBackground)
{
    WNDCLASS wc;
    wc.style=wc.cbClsExtra=wc.cbClsExtra=0;
    wc.lpfnWndProc = Proc;
    wc.hInstance = hInst;
    wc.hIcon = LoadIcon(NULL,IDI_APPLICATION);
    wc.hCursor = LoadCursor(NULL, IDC_ARROW);
    wc.hbrBackground = (HBRUSH)(brBackground + 1);
    wc.lpszMenuName = NULL;
    wc.lpszClassName = szName;
    return(RegisterClass(&wc) != 0);
}
 
//описание функции окон
LRESULT CALLBACK WndProc(HWND hwnd,UINT msg,WPARAM wParam,LPARAM lParam)
{
    switch(msg)
    { case WM_DESTROY:{PostQuitMessage(0); return 0;} }
    return DefWindowProc(hwnd, msg, wParam, lParam);
}

return(RegisterClass(&wc) != 0);

return(RegisterClassEx(&wc) != 0);

WNDCLASSEX windowsclass;
windowsclass.cbSize = sizeof(WNDCLASSEX);
windowsclass.style = CS_VREDRAW|CS_HREDRAW|CS_OWNDC|CS_DBLCLKS;
windowsclass.lpfnWndProc = MainWinProc;
windowsclass.cbClsExtra = 0;
windowsclass.cbWndExtra = 0;
windowsclass.hInstance = hinstance;
windowsclass.hIcon = LoadIcon(NULL, IDI_APPLICATION);
windowsclass.hCursor = LoadCursor(NULL, IDC_ARROW);
windowsclass.hbrBackground = (HBRUSH)GetStockObject(GRAY_BRUSH);
windowsclass.lpszMenuName = NULL;
windowsclass.lpszClassName = "WINDOWSCLASS";
windowsclass.hIconSm = LoadIcon(NULL, IDI_APPLICATION);