此虚拟方法的预设实作会引发事件，如先前在本主题中所述。

The default implementation of this virtual method raises the event as described earlier in this topic.

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实作只应执行不是由预设实作执行的工作。

Implementations should only perform work that is not performed by the default implementation.

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介面和介面成员是抽象的，介面并未提供预设实作。

Interfaces and interface members are abstract; interfaces do not provide a default implementation.

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