openfiledialog   dlg   =   new   openfiledialog();  
   dlg.defaultext   =   "xls";  
   dlg.filter   =   "text   files(*.xls)|*.xls||";  
   if   (dlg.showdialog()   ==   dialogresult.ok)  
   {  
    string   filepath   =   dlg.filename;
    txtsourcefilename.text=filepath;
   }

'調用外部應用程序打開選擇的文件

string openfilename;
   openfilename=txtsourcefilename.text;
   processstartinfo pinfo = new processstartinfo();
   pinfo.useshellexecute = true;
    if (file.exists(openfilename))
    {
     pinfo.filename = openfilename;
     //啟動進程
     process p = process.start(pinfo);
    }