feature question

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From: Kamen Yotov (kamen_at_yotov.org)
Date: 02/10/04 

Date: Tue, 10 Feb 2004 02:12:32 -0500

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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