索引器和可空类型

    #region 索引器

    public class IndexClass
    {
        private int[] arr = new int[100];

        public int this[int index]
        {
            get
            {
                if (index < 0 || index >= 100)
                {
                    return 0;
                }
                else
                {
                    return arr[index];
                }
            }
            set
            {
                if (!(index < 0 || index >= 100))
                {
                    arr[index] = value;
                }
            }
        }
    }

    #endregion

可空类型可以表示其基础值类型正常范围内的值，再加上一个 null 值

int? num = null;