Manager m2 = new Manager();
m2.Name = "Manager M2";
IMammal m3 = m2;
Mammal m4 = (Mammal)m3; // Generates runtime casting exception

interface IMammal
{
void Breathe();
}

class Mammal : IMammal
{
public void Breathe() { }
}

So even though one can cast back to the simplest form (Manager -> IMammal)
but one can't cast forward to a more complex form (Mammal -> IMammal). There
maybe reasons to do this but logically to me taking a concrete object and
flattening it into the picture of an object seems well, wrong. To me the
interface concept is to mandate certain properties of a type of object. As
you offered - all mammals breathe - so you would want to make sure any
mammal you create is "stamped" from the IMammal template. But you would
never want to revert back to the template and as it appears you can't
anyway.

I'll read your patterns a bit more and see if there's light to be found.
Thanks,

Dave Cline



On 6/6/06, Joćo Paulo Carreiro <joaoc@...> wrote:
>
> >Here we've turned a manager into a pet.
> You only seem to have broken the laws of nature ( or more specifically the
> laws of typing in C#), but in reality you havent'.
>
> If you run your code you'll get a InvalidCastException.
>
> As why would anyone cast to an interface from a class, there are several
> reasons. One of the most common, is "to separate interface from
> implementation", and it's used on a lot of the GoF design patterns.
> http://dofactory.com/Patterns/Patterns.aspx
>
> So the class becomes a "implementation detail", and all on your client
> code
> all you care for is interfaces.
>
> After all, extending your example, if the IMammal interface had a method
> "Breath()", then as long as all you wanted is for your objects to breath
> you
> could use the IMammal interface.
>
>
> -----Original Message-----
> From: StrongTypes@yahoogroups.com <StrongTypes%40yahoogroups.com> [mailto:
> StrongTypes@yahoogroups.com <StrongTypes%40yahoogroups.com>] On
> Behalf Of Dave Cline
> Sent: 06 June 2006 02:54
> To: StrongTypes@yahoogroups.com <StrongTypes%40yahoogroups.com>
> Subject: Re: [StrongTypes] Casting a SortedList Generic Class to a generic
> Class of base type.
>
> Paulo,
> Ah, well, I stand corrected. Thank you. I realize I was wrong but, is
> there
> a valid reason why someone would want to cast down to an interface?
> Essentially turning a person into a painting?
>
> Manager m2 = new Manager();
> m2.Name = "Manager M2";
>
> IMammal m3 = m2;
>
> Pet pet3 = (Pet)m3;
>
> Pets.Add(pet3);
>
> Here we've turned a manager into a pet. Which is humorous but certainly
> illogical. Yet by using an interface as some intermediary template we've
> manage to confound the laws of nature.
>
> Do you have an example of how anyone would do this in a beneficial way?
>
> Dave Cline
>
> On 6/5/06, Joćo Paulo Carreiro
<joaoc@...<joaoc%40programando.net>>
> wrote:
> >
> > >Trying to cast an object to it's interface well, that would be like
> > trying to turn a person into a painting.
> > Dave, your anology is wrong. It's perfectly valid to cast an object to
> > an interface it supports. Inclusive you don't even need to explicitly
> > cast
> > it:
> >
> > IMammal m3 = m2;
> >
> > Only when going the other way around you need to cast:
> > Managers.Add((Manager)m3);
> >
> > Santiago, the problem you're having is that Generics in C# are
> > "invariants".
> > A List<Person> is completely unrelated with List<Manager>, although
> > Manager inherits from Person.
> >
> > Maybe this blog post will help you:
> > http://blogs.msdn.com/rmbyers/archive/2005/02/16/375079.aspx
> >
> > If all you care is to have a list with the same elements, than you can
> > just create a new list and copy the original list in.
> >
> > List<Person> persons = new List<Person>(Managers.ToArray());
> >
> > Best Regards,
> > Paulo Carreiro
> >
> >
> > -----Original Message-----
> > From: StrongTypes@yahoogroups.com
<StrongTypes%40yahoogroups.com><StrongTypes%40yahoogroups.com> [mailto:
> > StrongTypes@yahoogroups.com
<StrongTypes%40yahoogroups.com><StrongTypes%40yahoogroups.com>] On Behalf
> > Of santiago.perez@... <santiago.perez%40dot.state.fl.us><
> santiago.perez%40dot.state.fl.us>
> > Sent: 05 June 2006 20:10
> > To: StrongTypes@yahoogroups.com
<StrongTypes%40yahoogroups.com><StrongTypes%40yahoogroups.com>
> > Subject: Re: [StrongTypes] Casting a SortedList Generic Class to a
> > generic Class of base type.
> >
> > Hey Dave thanks for your help and insight into casting to an
> > Interface. I understand what you mean, and I follow your example to
> > the T. My only problem is that I want Cast a list of one type to it's
> > base which youdidn't have an example of, something like this:
> >
> > List<Person> Persons =Managers ;
> >
> > DO I need to specifically cast it as follows?:
> > List<Person> Persons =(List<Person>)Managers ;
> >
> > NO matter what I try I can't cast a list of a sublass to it's base.
> > This doesn't seem like something that MS wouldn't have thought about.
> >
> > Thanks again.
> >
> > Santiago Perez
> > Florida's Turnpike Enterprise
> > Programmer Analyst
> >
> > Pompano Operations
> > Ph 954-975-4855 ex 1127
> > Cell 954-444-9429
> >
> > "Dave Cline"
> > <davecline@gmail.
> > com> To
> > Sent by: StrongTypes@yahoogroups.com
<StrongTypes%40yahoogroups.com><StrongTypes%40yahoogroups.com>
>
> > StrongTypes@yahoo cc groups.com Subject
> > Re: [StrongTypes] Casting a
> > 06/04/2006 06:08 SortedList Generic Class to a PM generic Class of
> > base type.
> >
> >
> > Please respond to
> > StrongTypes@yahoo
> > groups.com
> >
> >
> >
> > I'm not expert, in anybody's book, but I believe you're trying to use an
> > interface like a base class. An interface is really just a signature, a
> > template if you will. It does not represent inheritance. Trying to cast
> an
> > object to it's interface well, that would be like trying to turn a
> person
> > into a painting. It can't be done without breaking a few laws of nature
> > not
> > to mention Newtonian physics. But you can turn a person into a child, or
> > senior citizen or mammal or vertebrate. A picture of a person is really
> > just
> > a plan, a image of a three dimensional entity.
> >
> > Try not using an interface as a generics type. Below was my testing rig.
> >
> > But I maybe way off here.
> >
> > DC
> >
> > //~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > using System;
> > using System.Collections.Generic;
> > using System.Text;
> >
> > namespace InterfaceTesting
> > {
> > class Program
> > {
> > static void Main(string[] args)
> > {
> > System.Console.WriteLine("Testing Inheritance");
> >
> > Manager m1 = new Manager();
> > m1.Name = "Manager M1";
> > System.Console.WriteLine("1:" + m1.Name);
> >
> > Person p1 = (Person)m1;
> > System.Console.WriteLine("2:" + p1.Name);
> >
> > List<Manager> Managers = new List<Manager>();
> >
> > List<Person> Persons = new List<Person>();
> >
> > List<Pet> Pets = new List<Pet>();
> >
> > Manager m2 = new Manager();
> > m2.Name = "Manager M2";
> >
> > Managers.Add(m2);
> >
> > Persons.Add(m2);
> >
> > //Pets.Add(m2); // although Pet implements IMammal you can't
> > turn a manager into a pet - as much as we'd like to sometimes.
> >
> > System.Threading.Thread.CurrentThread.Suspend();
> > }
> > }
> >
> > interface IMammal
> > {
> > }
> >
> > class Pet : IMammal
> > {
> > public string Name = String.Empty;
> > }
> >
> > class Person : IMammal
> > {
> > public string Name = String.Empty;
> > }
> >
> > class Manager : Person
> > {
> > public string Title = String.Empty;
> > }
> > }
> >
> >
> > Yahoo! Groups Links
>
> >
> >
> >
>
> --
> Dave Cline
> www.davecline.com/
> davecline@... <davecline%40gmail.com>
>
> [Non-text portions of this message have been removed]
>
> Yahoo! Groups Links
>
>
>



--
Dave Cline
www.davecline.com/
davecline@...


[Non-text portions of this message have been removed]