设计模式：迭代器模式

迭代器模式（Iterator）：提供一种方法顺序访问一个聚合对象中各个元素，而又不暴露该对象的内部表示。

namespace IteratorDesign
{
    public abstract class Iterator
    {
        public abstract object First();
        public abstract object Next();
        public abstract bool IsDone();
        public abstract object CurrentItem();
    }
    public abstract class Aggregate
    {
        public abstract Iterator CreateIterator();
    }
    public class ConcreteAggregate : Aggregate
    {
        private IList<object> items = new List<object>();
        public override Iterator CreateIterator()
        {
            return new ConcreteIterator(this);
        }
        public int Count
        {
            get { return items.Count; }
        }
        public object this[int index]
        {
            get { return items[index]; }
            set { items.Insert(index, value); }
        }
    }
    public class ConcreteIterator : Iterator
    {
        private ConcreteAggregate aggregate;
        private int current = 0;
        public ConcreteIterator(ConcreteAggregate aggregate)
        {
            this.aggregate = aggregate;
        }
        public override object CurrentItem()
        {
            return aggregate[current];
        }

        public override object First()
        {
            return aggregate[0];
        }

        public override bool IsDone()
        {
            return current >= aggregate.Count ? true : false;
        }

        public override object Next()
        {
            object ret = null;
            current++;
            if(current<aggregate.Count)
            {
                ret = aggregate[current];
            }
            return ret;
        }
    }
}

测试代码：

            ConcreteAggregate a = new ConcreteAggregate();
            a[0] = "A";
            a[1] = "B";
            a[2] = "C";
            Iterator i = new ConcreteIterator(a);
            object item = i.First();
            while(!i.IsDone())
            {
                Console.WriteLine(i.CurrentItem());
                i.Next();
            }