C#中子类无法调用基类事件的解决方法

在基类中定义一个事件，在子类中引用会报

Error 1 The event ‘***’ can only appear on the left hand side of += or -= (except when used from within the type ' ***')

解决方法：

http://stackoverflow.com/questions/253757/why-events-cant-be-used-in-the-same-way-in-derived-classes-as-in-the-base-class