这是的IEnumerable＆LT; T＆GT; 是只是元素的顺序;认为这是一个仅正向游标。因为很多这些序列是产生价值的数据，或从数据库中记录集流，这是没有意义的添加的项目给他们。

An IEnumerable<T> is just a sequence of elements; see it as a forward only cursor. Because a lot of those sequences are generating values, streams of data, or record sets from a database, it makes no sense to Add items to them.