Because an event is a pointer to an event handler, the type of the event property must be a closure type. Similarly, any code to be used as an event handler must be an appropriately typed method of a class. 

To be compatible with an event of a given type, an event-handler method must have the same number and type of parameters, in the same order, passed in the same way. 

C++Builder defines closures for all its standard events. When you create your own events, you can use an existing closure if that is appropriate, or define one of your own.