<?xml version="1.0" encoding="utf-8" ?>
<configuration>
 <configSections>
  <section name="SQLConfiguration" type="ConfigurationDemo.SQLConfiguration,ConfigurationDemo"/>
  <section name="AccountConfiguration" type="ConfigurationDemo.AccountConfiguration,ConfigurationDemo"/>
 </configSections>
 <SQLConfiguration type="MSSQL" connectionString="server=.;integrated security=sspi;database=Northwind"></SQLConfiguration>
 <AccountConfiguration>
  <users username="liunian" password="123456"></users>
 </AccountConfiguration>
 <system.net>
  <mailSettings>
   <smtp from="liunian@qq.com">
    <network />
   </smtp>
  </mailSettings>
 </system.net>
</configuration>

  class SQLConfiguration : ConfigurationSection
  {
    [ConfigurationProperty("type", IsRequired = true)]
    public string Type
    {
      get { return this["type"].ToString(); }
      set { this["type"] = value; }
    }

    [ConfigurationProperty("connectionString", IsRequired = true)]
    public string ConnectionString
    {
      get { return this["connectionString"].ToString(); }
      set { this["connectionString"] = value; }
    }
  }

      SQLConfiguration sqlConfig = (SQLConfiguration)ConfigurationManager.GetSection("SQLConfiguration");
      Console.WriteLine(sqlConfig.Type);
      Console.WriteLine(sqlConfig.ConnectionString);

  public class AccountConfiguration : ConfigurationSection
  {
    [ConfigurationProperty("users", IsRequired = true)]
    public AccountSectionElement Users
    {
      get { return (AccountSectionElement)this["users"]; }
    }
  }

  public class AccountSectionElement : ConfigurationElement
  {
    [ConfigurationProperty("username", IsRequired = true)]
    public string UserName
    {
      get { return this["username"].ToString(); }
      set { this["username"] = value; }
    }

    [ConfigurationProperty("password", IsRequired = true)]
    public string Password
    {
      get { return this["password"].ToString(); }
      set { this["password"] = value; }
    }
  }

     AccountConfiguration accountConfig = (AccountConfiguration)ConfigurationManager.GetSection("AccountConfiguration");
      Console.WriteLine(accountConfig.Users.UserName);
      Console.WriteLine(accountConfig.Users.Password);

      Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
      SmtpSection section = config.GetSection("system.net/mailSettings/smtp") as SmtpSection;
      Console.WriteLine(section.From);