public class A
{
 public List<int> item { get; set; }

 //我在A类里怎么能监视这个item的增减？
}  

 A a = new A();
 a.item.add(1);
 //调用的地方只执行这些代码不需要额外代码

    class Program
    {
        static void Main(string[] args)
        {
            A a = new A();
            a.AddEvent += new AddHandler(a_AddEvent);
            a.Add(123);
            a[1] = 1234;
        }

        static void a_AddEvent(string messsage,int value)
        {
            Console.WriteLine("{0} 添加了{1}",messsage,value.ToString());
        }
    }
    public delegate void AddHandler(string message, int value);
    public class A
    {
        public event AddHandler AddEvent = null;
        private List<int> item = new List<int>();
        public int this[int index]
        {
            get {
                return this.item[index];
            }
            set
            {
                this.addValue("索引器",value);
            }
        }
        public void Add(int value)
        {
            this.addValue("A.Add",value);
        }
        private void addValue(string message,int value)
        {
            this.item.Add(value);
            if (this.AddEvent != null)
            {
                this.AddEvent(message,value);
            }
        }
        
    }