/// <summary>
 /// 创建当前集合的副本。
 /// </summary>
 /// <returns></returns>
 public DataControlFieldExCollection CloneFields()
 {
     DataControlFieldExCollection fields = new DataControlFieldExCollection();
     foreach (DataControlFieldEx field in this)
     {
         fields.Add(field.CloneField());
     }
     return fields;
 }