<connectionStrings>
      <add  name = "mdbconn" [String, Required, Collection Key]   connectionString = "" [String, Required]  providerName = "System.Data.SqlClient" [String]   />
</connectionStrings>

读取：
 string strConnection = ConfigurationManager.ConnectionStrings["mdbconn"].ToString();



第二种方法设置：
<appSettings>
    <add key="strcon" value="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=D:\Backup\我的文档\Visual Studio 2005\WebSites\4ec\App_Data\company.mdb"/>
  </appSettings>
 <connectionStrings/>

读取：
OleDbConnection con = new OleDbConnection(ConfigurationSettings.AppSettings["strcon"]);