SendMessage function
https://msdn.microsoft.com/en-us/library/windows/desktop/ms644950%28v=vs.85%29.aspx
系统预定义消息
https://msdn.microsoft.com/en-us/library/windows/desktop/ms644927%28v=vs.85%29.aspx#system_defined
使复选框按钮选中
#define BM_SETCHECK 0x00F1
#define BST_CHECKED 0x0001
SendMessage(hwnd, BM_SETCHECK, BST_CHECKED, 0);
原文：http://www.cnblogs.com/findumars/p/7433765.html