Types . covariance permits a method to have a more derived return type than what is defined in the delegate 协变允许将带有派生返回类型的方法用作委托，逆变允许将带有派生参数的方法用作委托。
2.
The major mode of network - based design is also studied and further more derive the safety problem of the design information during the process of delivering with the information 并对主要的网络化设计模式进行了研究并进而引出了设计信息交互过程中的安全性问题。
