A class icon is a picture that the system uses to represent a window of a particular class. An application can have two class icons — one large and one small. The system displays a window's large class icon in the task-switch window that appears when the user presses alt+tab, and in the large icon views of the task bar and explorer. The small class icon appears in a window's title bar and in the small icon views of the task bar and explorer.
To assign a large and small icon to a window class, specify the handles of the icons in the hIcon and hIconSm members of the WNDCLASSEX structure. The icon dimensions must conform to required dimensions for large and small class icons. For a large class icon, you can determine the required dimensions by specifying the SM_CXICON and SM_CYICON values in a call to the GetSystemMetrics function. For a small class icon, specify the SM_CXSMICON and SM_CYSMICON values. For information, see Icons.
If an application sets the hIcon and hIconSm members of the WNDCLASSEX structure to NULL, the system uses the default application icon as the large and small class icons for the window class. If you specify a large class icon but not a small one, the system creates a small class icon based on the large one. However, if you specify a small class icon but not a large one, the system uses the default application icon as the large class icon and the specified icon as the small class icon.
You can override the large or small class icon for a particular window by using the WM_SETICON message. You can retrieve the current large or small class icon by using the WM_GETICON message.