我的类是这样写的，但是系列化结果不符以上要求

public class DropDownList
 {  
  [XmlElement(typeof(Selection), ElementName = "Selection")]
  
  public ArrayList selections = new ArrayList();
  
  public DropDownList()
  {
   //
   // TODO: Add constructor logic here
   //
  }

  [XmlAttribute]
  public string  ID ="";
  private string _title;
  
  [XmlAttribute]
  public  string  Title
  {
  
   set
   {
    _title = value;
    ToolTip = value;
   }
   get
   {
    return _title;
   }
  }

  [XmlAttribute]
  public string  align ="Center";

  [XmlAttribute]
  public string  bgColor = "#00ff99";

  [XmlAttribute]
  public string  ToolTip;

  [XmlAttribute]
  public string  ReadOnly = "F";
       
  [XmlAttribute]
  public string num;

 }

  selection类是这样写的
   public class Selection
 {
  public Selection()
  {
   //
   // TODO: 在此处添加构造函数逻辑
   //
  
  }
      
  
  [XmlAttribute]
  public string Key;

  public string content;
  
 }

把selection逐个添加到dropdownlist的selection中，系列化dropdownlist后结果是：
   <DropDownList ID="" align="Center" bgColor="#00ff99" ReadOnly="F" num="3">
          <Selection Key="女">
            <content>女</content>
          </Selection>
          <Selection Key="男">
            <content>男</content>
          </Selection>
        </DropDownList>

改怎样改正？各位大虾，希望不吝赐教。