如何在web.config中定义和枚举列表

ASP.NET MVC4应用程序需要从web.config文件中读取列表。

web.config包含

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <configSections>
    <sectionGroup name="connectionGroup">
      <section
        name="connection"
        type="Helpers.connectionSection"
        allowLocation="true"
        allowDefinition="Everywhere"
      />
    </sectionGroup>
  </configSections>
  <connectionGroup>
    <connection url="site1" database="db1" />
    <connection url="site2" database="db2" />
  </connectionGroup>

枚举它的代码是：

        IList<ConnectionSection> connection =
                (IList<ConnectionSection>)ConfigurationManager.GetSection("connectionGroup");
        foreach (var s in connection)
            Debug.WriteLine(s.Url);

运行它会产生错误

Parser Error Message: Sections must only appear once per config file.  See the help topic <location> for exceptions.
Source Error: 
Line 19:   <connectionGroup>
Line 20:     <connection url="site1" database="site1" />
Line 21:     <connection url="site2" database="site2" />
Line 22:   </connectionGroup>

如何修复此问题，使web.config可以包含多个连接元素？

章节定义为：

namespace Helpers
{
    public class ConnectionSection : ConfigurationSection
    {
        [ConfigurationProperty("url", IsRequired = true, IsKey = true)]
        public string Url
        {
            get { return (string)this["url"]; }
            set { this["url"] = value; }
        }
        [ConfigurationProperty("database", DefaultValue = "mydb", IsRequired = false)]
        public string Database
        {
            get { return (string)this["database"]; }
            set { this["database"] = value; }
        }
   }
  }

public class ConnectionGroupConfigurationSection : ConfigurationSection
{
    public const string NodeName = "connectionGroup";
    [ConfigurationProperty("", IsDefaultCollection=true)]
    public ConnectionGroupConfigurationElementCollection ConnectionGroup
    {
        get { return (ConnectionGroupConfigurationElementCollection)this[""]; }
    }
}
public class ConnectionGroupConfigurationElementCollection : ConfigurationElementCollection
{
    protected override ConfigurationElement CreateNewElement()
    {
        return new ConnectionElement();
    }
    protected override object GetElementKey(ConfigurationElement element)
    {
        return (element as ConnectionElement).Id;
    }
    public override ConfigurationElementCollectionType CollectionType
    {
        get { return ConfigurationElementCollectionType.BasicMap; }
    }
    protected override string ElementName
    {
        get { return "connection"; }
    }
}
public class ConnectionElement : ConfigurationElement
{
    [ConfigurationProperty("url", IsRequired=true, IsKey=true)]
    public string Url { get { return (string)this["url"]; } }
    [ConfigurationProperty("database", DefaultValue = "mydb", IsRequired = false)]
    public string Database { get { return (string)this["database"]; } set { this["database"] = value; } }
}