WNDCLASS: The WNDCLASS structure contains the window class attributes that are registered by the RegisterClass function.


typedef struct _WNDCLASS { 
    UINT    style; 
    WNDPROC lpfnWndProc; 
    int     cbClsExtra; 
    int     cbWndExtra; 
    HANDLE  hInstance; 
    HICON   hIcon; 
    HCURSOR hCursor; 
    HBRUSH  hbrBackground; 
    LPCTSTR lpszMenuName; 
    LPCTSTR lpszClassName; 

style - Specifies the class style(s). Styles can be combined by using the bitwise OR (|) operator. This member can be any combination of the following values:
lpfnWndProc - Pointer to the window procedure. You must use the CallWindowProc function to call the window procedure. For more information, see WindowProc.
cbClsExtra - Specifies the number of extra bytes to allocate following the window-class structure. The system initializes the bytes to zero.
cbWndExtra - Specifies the number of extra bytes to allocate following the window instance. The system initializes the bytes to zero. If an application uses WNDCLASS to register a dialog box created by using the CLASS directive in the resource file, it must set this member to DLGWINDOWEXTRA.
hInstance - Handle to the instance that the window procedure of this class is within.
hIcon - Handle to the class icon. This member must be a handle of an icon resource. If this member is NULL, an application must draw an icon whenever the user minimizes the application's window.
hCursor - Handle to the class cursor. This member must be a handle of a cursor resource. If this member is NULL, an application must explicitly set the cursor shape whenever the mouse moves into the application's window.
hbrBackground - Handle to the class background brush. This member can be a handle to the physical brush to be used for painting the background, or it can be a color value. A color value must be one of the following standard system colors (the value 1 must be added to the chosen color). If a color value is given, you must convert it to one of the following HBRUSH types:
The system automatically deletes class background brushes when the class is freed. An application should not delete these brushes, because a class may be used by multiple instances of an application.
When this member is NULL, an application must paint its own background whenever it is requested to paint in its client area. To determine whether the background must be painted, an application can either process the WM_ERASEBKGND message or test the fErase member of the PAINTSTRUCT structure filled by the BeginPaint function.
lpszMenuName - Pointer to a null-terminated character string that specifies the resource name of the class menu, as the name appears in the resource file. If you use an integer to identify the menu, use the MAKEINTRESOURCE macro. If this member is NULL, windows belonging to this class have no default menu.
lpszClassName - Pointer to a null-terminated string or is an atom. If this parameter is an atom, it must be a global atom created by a previous call to the GlobalAddAtom function. The atom, a 16-bit value, must be in the low-order word of lpszClassName; the high-order word must be zero.
If lpszClassName is a string, it specifies the window class name.

The following predefined control classes can be specified in the lpClassName parameter.

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