说明：  索引器（C# 编程指南） 索引器允许类或结构的实例就像数组一样进行索引。索引器类似于属性，不同之处在于它们的访问器采用参数。
(Indexer (C# Programming Guide) indexer permit or structure, like for instance as an array index. Indexer is similar to attributes, the only difference is that they used the parameters of the accessor.)