Code
//这个地方的this[""]代表什么意思？
[ConfigurationProperty("", IsDefaultCollection = true)]
public UrlConfigCollection Uris
{
    get{
        return (UrlConfigCollection)this[""];
    }
}

//这里是UrlConfigCollection类的实现
class UrlConfigCollection : ConfigurationElementCollection<string, UrlConfigElement>
{
  public override ConfigurationElementCollectionType CollectionType
  {
    get { 
       return ConfigurationElementCollectionType.AddRemoveClearMap; 
     }
  }
  protected override string ElementName
  {
      get { return "urls"; }
  }
  
  protected override string GetElementKey(UrlConfigElement element)
  {
      return element.Name;
   }
}