One of the most important things you do with such a family of classes is to treat an object of a derived class as an object of the base class.

你对这个家族类能做的最重要的一件事就是把衍生类当作一个基类去对待。

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Although sometimes you may want an event that can only be raised by the base class, most of the time, you should enable the derived class to invoke base class events.

虽然有时您可能会想要使用只能由基底类别所引发的事件，但是在多数情形下，您应该启用衍生类别来叫用基底类别事件。

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When overridden in a derived class, gets the current item as a boxed object of the most appropriate.

当在派生类中重写时，根据当前项的架构类型以最合适的。

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