调试易

看书时书上习题答案的代码
 public class ShortCollection<T>:IList<T>
    {
        protected Collection<T> innerCollection;
        protected int maxSize = 10;
        public ShortCollection()
            : this(10)             
        {
        }
        public ShortCollection(int size)
        {
            maxSize = size;
            innerCollection = new Collection<T>();
        }
        public ShortCollection(List<T> list)
            : this(10, list)            
        {
        }
那两个this时怎么个意思？？？
  public T this[int index]            
        {
            get
            {
                return (innerCollection as IList<T>)[index];
            }
            set
            {
                (innerCollection as IList<T>)[index] = value;
            }
        }
还有这个
实在理解不能 求指导