http://www.tiantiansoft.com/bbs/dispbbs.asp?boardID=40&ID=80348

private bool HaveOpened(Form myMdi,string windowName)
  {
   //查看窗口是否已经被打开
   bool bReturn=true;
   for(int i=0;i<myMdi.MdiChildren.Length;i++)
   {
    //MessageBox.Show(myMdi.MdiChildren[i].Name);
    if(myMdi.MdiChildren[i].Name==windowName)
    {
     myMdi.MdiChildren[i].BringToFront();
     bReturn=false;
     break;
    }
   }
   return bReturn;
  }

打开窗口的时候：

ProcessesManage w0=new ProcessesManage();
     if(HaveOpened(myMdi,"ProcessesManage"))
     {
      w0.MdiParent=myMdi;
      w0.Show();
     }

这里的myMdi，是Form类型的，传递的是mdi窗口