I've extended the article "Covariance and Contravariance in Delegates" from
the C# Programmers' Guide, and it seems contravariance of delegates is
implemented wrong.
Lambda calculus states that Handler2Method is a subtype of HandlerMethod...
why can't I cast (it should be implicit conversion, actually)? However the
last line does work, but I'm not wanting
to create new delegate objects. I'm actually implementing contravariant
events, and I'm scared about my remove implementation. In
event BaseInterface.baseEvent {
add { derivedEvent += new DerivedDelegate(value); } // add a new
handler, that's ok
remove { derivedEvent -= new DerivedDelegate(value); } // I don't want
to remove a new handler, I want to remove the one I added on the line above!
}
Is removal going to work, or have I created a "new" object that won't match
the existing one????
HELP!
class Mammals
{
}
class Dogs : Mammals
{
}
class Program
{
public delegate void HandlerMethod(Dogs sampleDog);
public delegate void Handler2Method(Mammals sample);
public static void FirstHandler(Mammals elephant)
{
}
public static void SecondHandler(Dogs sheepDog)
{
}
static void Main(string[] args)
{
// Contravariance permits this delegate.
HandlerMethod handler1 = FirstHandler;
HandlerMethod handler2 = SecondHandler;
Handler2Method handler3 = FirstHandler; // ok
HandlerMethod handler4 = handler3; // error CS0029: Cannot
implicitly convert type 'Program.Handler2Method' to 'Program.HandlerMethod'
HandlerMethod handler5 = (HandlerMethod)handler3; // error CS0030:
Cannot convert type 'Program.Handler2Method' to 'Program.HandlerMethod'
HandlerMethod handler6 = new HandlerMethod(handler3); // ok
}
}
