#define UNIQE_NAME "{1AB792D6-EAF2-3267-9A84-9135681127A4}"
#define GIS_MSG "{D48CA993-4925-41cb-8F59-ABAAAFCEF797}"

const UINT ID_GIS_INSTANCE = ::RegisterWindowMessage(GIS_MSG); //注册消息

// 确保程序只有一个实例在运行
 m_hOneInstance = ::CreateMutex(NULL, FALSE, UNIQE_NAME);
 if (GetLastError() == ERROR_ALREADY_EXISTS )//如果存在前一个
 {
  //发送消息，激活实例
  DWORD dwRecipients = BSM_APPLICATIONS; 
  ::BroadcastSystemMessage(BSF_NOHANG, &dwRecipients,
   ID_GIS_INSTANCE, // registered window message
   0, 0);    // user defined parameters

  return FALSE;
 }

ON_REGISTERED_MESSAGE(ID_GIS_INSTANCE, OnGISInstance)
LRESULT CMainFrame::OnGISInstance(WPARAM wParam, LPARAM lParam)
{
 theApp.m_pMainWnd->ShowWindow(SW_SHOWMAXIMIZED);
 theApp.m_pMainWnd->SetForegroundWindow();

 ::AfxMessageBox("本程序已经有一个实例在运行了。");

 return 0;
}

本文转自莫水千流博客园博客，原文链接：http://www.cnblogs.com/zhoug2020/archive/2012/04/02/2430101.html，如需转载请自行联系原作者