wndclass.hIcon = Win32.LoadIcon(IntPtr.Zero, new IntPtr((int)SystemIcons.IDI_APPLICATION));

To load system icons, provide IntPtr.Zero for hInstance, and use one of the values of SystemIcons for lpIconName

public enum SystemIcons

     Public Enum SystemIcons

System.Drawing.SystemIcons, although note that not all icons are included and at least the Shield icon is incorrect on Win 7 / Win 2008.