delegate contravariance not fully implemented

  • Thread starter Thread starter Ben Voigt
  • Start date Start date
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Ben Voigt

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 convert? 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
}
}
 
Ben Voigt said:
I've extended the article "Covariance and Contravariance in Delegates"
from the C# Programmers' Guide, and it seems contravariance of delegates
is implemented wrong.
Click to expand...

Obviously I meant to put this in the C# newsgroup.... moving my question
there.
Lambda calculus states that Handler2Method is a subtype of
HandlerMethod... why can't I convert? 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
}
}
Click to expand...
 
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