C# 4 delegates contravariance
A little quiz.
Given the following two delegates:
delegate void EventHandler(object sender, EventArgs e);
delegate void PropertyChangedEventHandler(object sender, PropertyChangedEventArgs e);
Does the following code compile?
void propertyHandler(object sender, PropertyChangedEventArgs e)
{ }
EventHandler handler = propertyHandler;
Did you answer "YES, because PropertyChangedEventHandler IS an EventHandler"?
Unfortunately, the answer is no. Imagine how would the following code work:
handler(this, new EventArgs());
How could just EventArgs be passed to propertyHandler, which expects PropertyChangedEventArgs?
Actually, the exact opposite is correct in C# 4:
void handler(object sender, EventArgs e)
{ }
PropertyChangedEventHandler propertyHandler = handler;
Then, calling
propertyHandler(this, new PropertyChangedEventArgs("Name"))
is perfectly ok, because the handler just sees passed PropertyChangedEventArgs as EventArgs.
So the conclusion is: you can handle specialized events using less specialized handlers (you can handle eg. PropertyChanged event using just an EventHandler).
For more info, see C# delegates on msdn.
