If marked as out, the interface supports co-variance and the given type can only appear as a return type or read-only property.
如果标记为out，那么接口就支持协变，并且给定的类型只能表现为一个返回类型或只读属性。
2
Up last is support for Co- and Contra-Variance.
最后就是对协变和逆变的支持了。
3
Rice moisture content, film of different kinds and co 2 content were measured, the orthogonal test of rice storage with film packing was carried out, and its variance was analysed.