如果标记为in，那么类型只能表现为一个参数或只写属性。

If marked in, the type can only appear as a parameter or set-only property.

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属性提供特定于操作类型的事件参数。

Property supplies the event arguments that are specific to the type of action.

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属性提供特定于操作类型的事件参数。

Property supplies the event arguments that are specific to the type of action.

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