您可以修改已有类，将它作为参数传递，并利用 API，而不需要基础 API 声明一个抽象类或接口。

You can modify an existing class, pass it as a parameter, and take advantage of the API without requiring the base API to declare an abstract class or interface.

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如果父类是抽象类且至少包含一个抽象成员，则所有这些成员都将作为非抽象的继承类实现。

If the parent class is an abstract class and if it contains at least one abstract member, then all abstract members will be implemented as non-abstract inheriting classes.

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Dictionary是一个抽象类，可以作为某些映射key与值的类的父类，比如Hashtable。

The Dictionary class is the abstract parent of any class, such as Hashtable, which maps keys to values.

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