若类别成员、方法、栏位、属性或事件是位于覆写基底类别虚拟成员的衍生类别上，则可宣告为密封。
A class member, method, field, property, or event, on a derived class that is overriding a virtual member of the base class can declare that member as sealed.
衍生类别中的函式试图覆写基底类别中的虚拟函式，但衍生类别函式没有与基底类别函式相同的传回型别。
A function in a derived class attempted to override a virtual function in a base class, but the derived class function did not have the same return type as the base class function.
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