Re: feature question
From: Kamen Yotov (kamen_at_yotov.org)
Date: 02/11/04
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Date: Tue, 10 Feb 2004 23:35:49 -0500
There is a misunderstanding.
When I say "A -> B", I mean "B derives from A".
It is not possible "A -> B" and "Q -> B", at least in C#.
Let me know if you still thing you have a case...
Kamen
"Stephen Martin" <smartin@removethis.emsoft.andthis.ca> wrote in message
news:%23QJbEP87DHA.2168@TK2MSFTNGP12.phx.gbl...
> While the return type doesn't present a problem since type D is always
also
> type C, the input parameter is a problem. Your delegate DF is a guarantee
> that you can call that method with a parameter of type B or any subclass
of
> B.
>
> If I have type Q -> B and you pass me a delegate d of type DF but it can
> only accept a parameter of type A then it will compile fine but when I try
> to invoke the delegate with a parameter of type Q I will get a nasty
runtime
> error because you've broken the contract that I can use any subclass of B
> with that delegate.
>
> There is also a similar problem with overrides. The base class guarantees
> that the method can be called with any subclass of the parameter type but
> the override narrows that guarantee so the method can no longer be used
> polymorphically.
>
> I think that overriding the return types is simpler since it doesn't, in
> fact, violate the interface contract and I'm sure overriding the parameter
> types could be done but I'm not sure it would be desirable and I don't
think
> it would be simple.
>
> "Kamen Yotov" <kamen@yotov.org> wrote in message
> news:eYsjZV67DHA.2952@TK2MSFTNGP09.phx.gbl...
> > hi all,
> > i first posted this on
> > http://msdn.microsoft.com/vcsharp/team/language/ask/default.aspx
> > (ask a c# language designer) a couple of days ago, but no response so
> far...
> > therefore, i am pasting it here as well... enjoy!
> >
> > (you can skip to the source at the end of the message if you like...)
> >
> > Consider:
> >
> > 1) a class relationships A -> B and C -> D
> > 2) a delegate declaration "delegate C DF (B b);"
> > 3) a function with signature "D F (A a);"
> >
> > According to C# syntax and semantics, an instance of the delegate DF
> > cannot be created from the function F, as the parameter types and the
> > return type do not match the delegate declaration.
> >
> > I would argue that from formal language type theory, it should be
> > possible, because of polymorphism.
> >
> > Suppose this hypothetic code:
> >
> > {
> > DF d = new DF(F);
> >
> > B b = new ...
> >
> > C c = d(b);
> > }
> >
> > I have no reasons to believe that such code should break. In fact:
> >
> > 1) d is an instance of DF using function F
> >
> > 2) d is called with parameter b which is of type B, which derives from
> > A,
> > therefore F can be called itself with b
> >
> > 3) d's result is assigned to c, which is of type C, which type D derives
> > from. Because F returns D, it's result is assignable to C-typed
> > objects...
> >
> > In summary I would argue that it is type-safe to allow this. Further I
> > don't think that it adds any further complexity in the language and it
> > is something worth having. I personally expected to see it there, was
> > disappointed when it I did not, and I have needed it a number of times.
> >
> > One fact that confirms my thesis is that if we write the following
> > function:
> >
> > C auxF (B b)
> > {
> > return F(b);
> > }
> >
> > The new auxF is compatible with the delegate and does exactly what we
> > want.
> > To refine my question further I would say: Why do we need auxF???
> >
> > Please find a "working" C# program below that illustrates the issue!
> >
> > Please outline the reasons for which this is not part of the C#
> > language.
> > Do not hesitate to contact me personally if you need more information
> > regarding this question.
> >
> > Best Regards,
> > Kamen Yotov
> > kamen@yotov.org
> > http://yotov.org
> >
> > using System;
> >
> > class A {}
> > class B: A {}
> > class C {}
> > class D: C {}
> >
> > delegate C DF (B b);
> >
> > class Run
> > {
> > static D F (A a)
> > {
> > return new D();
> > }
> >
> > static C auxF (B b) // I would like to get rid of this function
> > {
> > return F(b);
> > }
> >
> > static void Main(string[] args)
> > {
> > DF d = new DF(auxF); // I want "new DF(F)" here!
> > B b = new B();
> >
> > Console.WriteLine(d(b));
> > }
> > }
> >
> >
> >
> >
>
>
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