<connectionStrings>
      <add name="GPD" connectionString="data source=172.16.161.*;User ID=sa;pwd=test;Initial Catalog=GPD" />
    </connectionStrings>

<configuration>
    <appSettings>
        <add key="conStr" value="Server=.;Database=ETer;uid=sa;pwd=sa"/>
    </appSettings>
    <connectionStrings/>

------解决方案--------------------
C# code

//添加引用
using System.Configuration;

//获取配置文件连接字符串
String connStr = ConfigurationManager.ConnectionStrings["connectionString"].ToString();

------解决方案--------------------
//添加引用
using System.Configuration;

//获取配置文件连接字符串
String connStr = ConfigurationManager.ConnectionStrings["GPD"].ToString();



------解决方案--------------------
配置文件中：
   </connectionStrings>
<connectionStrings>
     <add name="GPD" value="data source=172.16.161.*;User ID=sa;pwd=test;Initial Catalog=GPD" />
   </connectionStrings>

CS中
//添加引用  
using System.Configuration;  

//获取配置文件连接字符串  
String connStr = ConfigurationManager.ConnectionStrings["GPD"].ToString();  



------解决方案--------------------
string connstr=System.Configuration.ConfigurationSettings.AppSettings["GPD"];