public static string Config(string key) 
       { 
           ExeConfigurationFileMap file = new ExeConfigurationFileMap(); 
           file.ExeConfigFilename = @"Providers\\Provider.config"; 
           Configuration config = ConfigurationManager.OpenMappedExeConfiguration(file, ConfigurationUserLevel.None); 
           AppSettingsSection appsection = (AppSettingsSection)config.GetSection("appSettings"); 
           return appsection.Settings[key].Value; 
       } 

<?xml version="1.0" encoding="utf-8" ?> 
<configuration> 
    <appSettings> 
        <add key="ConnectionString" value="Server=(local);Database=DB;User Id=sa;Password=123" /> 
    </appSettings> 
</configuration> 

//里面的参数为配置文件的key  
string strConn=Config("ConnectionString");