ConfigurationManager

 <!-- App.config, Web.config 应用程序配置文件 -->

<?xml version="1.0" encoding="utf-8"?>
<configuration>
<appSettings>
   <add key="ServiceName" value="MSSQLSERVER"/> <!-- SQL2005 "MSSQL$SQLEXPRESS"-->
</appSettings>
<connectionStrings>
<add name="SQL2000" connectionString="Data Source=.; DataBase=ProductSet; Integrated Security=True;" />
</connectionStrings>
<!-- SQL 2005, 2008
<connectionStrings>
   <add name="DSN" connectionString="Data Source=./SQLEXPRESS; AttachDBFilename=|DataDirectory|/ProductSet.mdf; Integrated Security=True; User Instance=True"/>
</connectionStrings>
   <connectionStrings>
        <add name="Access2003" connectionString="Provider=Microsoft.Jet.OLEDB.4.0;Data Source=|DataDirectory|/Image.mdb;Jet OLEDB:Database Password=jinzhexian"
            providerName="System.Data.OleDb" />
        <add name="Access2007" connectionString="Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|/User.accdb;Jet OLEDB:Database Password=jinzhexian"
            providerName="System.Data.OleDb" />
    </connectionStrings>
-->
</configuration>

using System.Configuration;

string sql = ConfigurationManager.ConnectionStrings["DSN"].ConnectionString;

string serviceName = ConfigurationManager.AppSettings["ServiceName"];