public Event AddEvent(Event model)
 {            
     if (model!=null)
     {
         model.Id = _nextId++;
         model.Time = DateTime.Now;
        events.Add(model);
     }
     return model;
 }
 /// <summary>
 /// 更新
 /// </summary>
 /// <param name="item"></param>
 /// <returns></returns>
 public bool Update(Event item)
 {
     if (item ==null)
     {
         throw  new ArgumentException("item");
     }
     int index = events.FindIndex(e => e.Id == item.Id);
     if (index ==-1)
     {
         return false;
     }
     events.RemoveAt(index);
     events.Add(item);
     return true;
 }