If a class that uses an abstraction has to downcast to a specific subclass in one section, you're not following the Open Closed Principle.
如果在某一部分中，一个抽象了的类必须向下转换为特定的子类，那么你就没有遵循开闭原则。
At the top of the VFS is a common API abstraction of functions such as open, close, read, and write.
在VFS上面，是对诸如open、close、read和write之类的函数的一个通用API抽象。
TCP further provides the port abstraction, which enables a host to open multiple TCP connections in parallel.
TCP进一步提供了端口抽取功能，这使得主机可以并行地打开多个 TCP连接。
And, MDT can open IGES and STEP file of other CAD systems. In this way, the drawing can realize the abstraction of information too in MDT in other CAD.
并且，MDT能导入其他CAD系统中的IGES和STEP图形文件，这样，其它CAD中的图形在MDT中也可以实现图形信息的提取。
And, MDT can open IGES and STEP file of other CAD systems. In this way, the drawing can realize the abstraction of information too in MDT in other CAD.
并且，MDT能导入其他CAD系统中的IGES和STEP图形文件，这样，其它CAD中的图形在MDT中也可以实现图形信息的提取。
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