public class XList<T> : IEnumerable, IEnumerator    {        #region List 简单实现        /// <summary>        /// 存储数据 数组        /// </summary>        private T[] _items;        /// <summary>        /// 默认容量        /// </summary>        private const int DefaultLength = 4;        /// <summary>        /// 插入最后一个元素索引（初始化时没有元素，所以为 -1）        /// </summary>        private int _lastIndex = -1;        /// <summary>        /// 无参构造        /// </summary>        public XList()        {            _items = new T[DefaultLength];        }        /// <summary>        /// 带初始容量的构造        /// </summary>        /// <param name="length"></param>        public XList(int length)        {            if (length <= 0)            {                throw new Exception("长度必须大于0");            }            _items = new T[length];        }        /// <summary>        /// 索引器        /// </summary>        /// <param name="index"></param>        /// <returns></returns>        public T this[int index]        {            get            {                CheckIndex(index); // 验证索引越界                return _items[index];            }            set { Insert(index, value); }        }        /// <summary>        /// 添加元素        /// </summary>        /// <param name="t"></param>        public void Add(T t)        {            Insert(_lastIndex + 1, t);        }        /// <summary>        /// 插入元素        /// </summary>        /// <param name="index"></param>        /// <param name="t"></param>        public void Insert(int index, T t)        {            if (index < 0)            {                throw new Exception("索引不能小于0");            }            KeepCapacity();            if (index <= _lastIndex) // 插入位置已有元素            {                T[] temp = new T[Capacity];                for (int i = 0; i < index; i++)                {                    temp[i] = _items[i];                }                temp[index] = t;                for (int i = index; i <= _lastIndex; i++)                {                    temp[i + 1] = _items[i];                }                _items = temp;            }            else if (index == _lastIndex + 1) // 在末尾插入             {                _items[++_lastIndex] = t;            }            else            {                throw new Exception("索引越界");            }        }        /// <summary>        /// 按 索引 移除元素        /// </summary>        /// <param name="index"></param>        public void Remove(int index)        {            CheckIndex(index);            if (index != _lastIndex) // 移除元素不是最后一个元素            {                T[] temp = _items;                for (int i = index; i < _lastIndex; i++)                {                    temp[i] = _items[i + 1];                }                _items = temp;            }            // 移除元素是最后一个元素，不做处理            _lastIndex--;        }        /// <summary>        /// 集合存储元素数量        /// </summary>        /// <returns></returns>        public int Count()        {            return _lastIndex + 1;        }        /// <summary>        /// 清空集合        /// </summary>        public void Clear()        {            _lastIndex = -1;        }        /// <summary>        /// 集合 容量        /// </summary>        private int Capacity        {            get            {                if (_items != null)                {                    return _items.Length;                }                return -1;            }        }        /// <summary>        /// 保证容量充足，如果数组容量不够就重置大小，容量扩大一倍        /// </summary>        private void KeepCapacity()        {            if (Capacity == -1) // 数组为 null 的情况            {                _items = new T[DefaultLength];                return;            }            if (_lastIndex < Capacity - 1) // 容量足够            {                return;            }            // 容量不够            int length = Capacity * 2;            T[] temp = new T[length];            for (int i = 0; i < this.Capacity; i++)            {                temp[i] = _items[i];            }            _items = temp;        }        /// <summary>        /// 检查索引越界的情况        /// </summary>        /// <param name="index"></param>        private void CheckIndex(int index)        {            if (index > _lastIndex)            {                throw new Exception("索引越界");            }        }        #endregion        #region 实现枚举        #region IEnumerable 成员        public IEnumerator GetEnumerator()        {            return this;        }        #endregion        #region IEnumerator 成员        private int _position = -1;        public object Current        {            get { return _items[_position]; }        }        public bool MoveNext()        {            if (_position < _lastIndex)            {                _position++;                return true;            }            return false;        }        public void Reset()        {            _position = -1;        }        #endregion        #endregion    }