当T：U？</u> </t>时，为什么Action <t>不可隐式转换为Action <u>

Question

考虑以下代码：

class BaseEvent {}
class DummyEvent : BaseEvent {}

void FooBase(BaseEvent e) {}
void FooDummy(DummyEvent e) {}

void main(String[] args) {
   Action<BaseEvent> actionBase = FooBase;
   Action<DummyEvent> actionDummy = FooDummy;

   // Works implicit
   Action<DummyEvent> actionDummy2 = actionBase;
   // Only explcit
   Action<BaseEvent> actionBase2 = (Action<BaseEvent>)actionDummy; 
}

我不明白为什么它会像这样工作，考虑它如何与对象引用一起工作，它会不会更有意义呢？

void main(String[] args) {
   BaseEvent baseEvent = new BaseEvent();
   DummyEvent dummyEvent = new DummyEvent();

   // Works implicit
   BaseEvent baseEvent2 = dummyEvent;     
   // Only explicit
   DummyEvent dummyEvent2 = (DummyEvent)baseEvent2; 
}

我想编写存储Action of Map的代码并传入不同的事件，如下所示：

class EventManager {
   Dicionary<int, Action<BaseEvent>> eventHandlers = new Dicionary<int, Action<BaseEvent>>();

   public void AddHandler(int eventType, Action<BaseEvent> eventHandler) {
      eventHandlers.Add(eventType, eventHandler);
   }
   public void TriggerEvent(BaseEvent e) {
      eventHandlers[e.EventType](e);
   }
}

void HandleDummyEvent(DummyEvent e) 
{
   // Some code
}

void main(String[] args) {
   EventManager eventManager = new EventManager();
   eventManager.AddHandler(DummyEvent.EventType, HandleDummyEvent);
   eventManager.TriggerEvent(new DummyEvent());
}