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Generic class and IEquatable problems

I want a generic class to implement the IEquatable<T> interface. The class has data items of type T. For the generic class to be equatable the data items need to be that as well.
Here is my generic class:
public class V<T> : IEquatable<V<T>> where T : IEquatable<T>
{
public V(T[] Value)
{
this.Value = Value;
}
T[] Value { get; set; }
public bool Equals(V<T> other)
{
if (Value.Count() != other.Value.Count()) return false;
for (int i = 0; (i < Value.Count()) && i < other.Value.Count(); i++)
{
if (!Value[i].Equals(other.Value[i])) return false;
}
return true;
}
}
And here is the problem.
When I compile the above generic class I get the following message.
GenericArguments[0], 'T' on 'Myspace.Generic.V`1[T]' violates the constraint of type parameter 'T'.
Where do I make the mistake in my reasoning or what is wrong with my generic class?
Note:
When I leave IEquatable<V<T>> out at the generic class and the code for public bool Equals(V<T> other) intact then the generic class compiles and is usable. Except for detecting IEquitable by the compiler.
public class V<T> where T : IEquatable<T>
{
The above code works but instances of V<T> are no longer recognised as IEquitable
Note2:
Thanks to dasblinkenlight for trying this code in a solution on its own I found out that it is most likely a configuration problem and not a coding problem. I now consider this specific question as answered but I've not yet identified my configuration problem.
Note3:
The actual cause of the problem is an NUnit test module that loads the dll through an accessor. Changing the test procedures is required but IEquatable<V<T>> is now used wihtout any problems.
Problem Solved.

There is nothing wrong with your generic class. Something is wrong with the class that you are passing as its generic parameter T. Namely, SomeClass, the class that you pass in V<SomeClass> does not implement IEquitable<SomeClass>.
Your V<T> class requires T to be an implementation of IEquitable<T>. You need it in order to check element-by-element equality of arrays using the Value[i].Equals(other.Value[i]) expression. If whatever class that you use as V<T>'s generic parameter is not equitable to itself, the compiler would complain.

How to use a class's type as the type argument for an inherited collection property in C#

I am trying to create a representation of various types of card that inherit from a generic card class and which all contain references to their owning decks.
I tried re-declaring them, as suggested here, but it still won't convert to the specific card type.
The code I currently have is as such:
public class Deck<T> : List<T>
where T : Card
{
void Shuffle()
{
throw new NotImplementedException("Shuffle not yet implemented.");
}
}
public class Card
{
public Deck<Card> OwningDeck { get; set; }
}
public class FooCard : Card
{
public Deck<FooCard> OwningDeck
{
get
{
return (Deck<FooCard>)base.OwningDeck;
}
set
{
OwningDeck = value;
}
}
}
The compile-time error I am getting:
Error 2 Cannot convert type Game.Cards.Deck<Game.Cards.Card> to Game.Cards.Deck<Game.Cards.FooCard>
And a warning suggesting I use a new operator to specify that the hiding is intentional. Would doing so be a violation of convention? Is there a better way?
My question to stackoverflow is this: Can what I am trying to do be done elegantly in the .NET type system? If so, can some examples be provided?

You could equip your cards with a generic parameter that specifies the base class of the cards you're using:
public class Card<TCard>
where TCard : Card<TCard>
{
public Deck<TCard> OwningDeck { get; set; }
}
Your FooCard class would then look like this:
public class FooCard : Card<FooCard>
An advantage over your current code could be that you don't have to redeclare the OwningDeck property; it's automatically of type Deck<FooCard> in FooCard.

What you are hitting on here, is a common design mistake, that you are trying to specialize the property 'OwningDeck' to FooCard, which is 'hiding' the base declaration of 'OwningDeck'.
The choice you have made, to put 'OwningDeck' in the Card class, is intended to ensure that all types of Card have an OwningDeck property - meaning also, that THAT property must be of a lowest-common-denominator Type (i.e. 'Deck').
As such, you can not redeclare this property in derived types (i.e. FooCard), because you are then attempting to CHANGE the type of the property - a bad design choice - and not actually supported in .net.
The reason you are getting a warning, is because, the property Deck<FooCard> OwningDeck HIDES the property Deck<Card> OwningDeck. If someone casts the variable to the type Card, then accessed the OwningDeck property, they will get the property from OwningDeck. If however, they cast it to FooCard then access the same property, they will get the value returned from Deck<FooCard> OwningDeck. The compiler doesnt know these may be accessing the same variable, but it is VERY risky, because a programmer will not expect this behavior from your code.
As far as a better implementation I see O.R. Mapper has beat me to the actual implementation. What he said ;)

You can achieve something close to what you're looking for if you are willing to make the Card class generic, like this:
public class Deck<T> : List<Card<T>>
where T : Card<T>
{
void Shuffle()
{
throw new NotImplementedException("Shuffle not yet implemented.");
}
}
public class Card<T> where T : Card<T>
{
public Deck<T> OwningDeck { get; set; }
}
public class FooCard : Card<FooCard>
{
}
The reason the original code doesn't work is that a Deck<Card> is not the same as a Deck<FooCard>, nor vice versa. Imagine the following:
Deck<Card> deck = new Deck<Card>();
deck.Add(new BarCard());
Deck<FooCard> fooDeck = (Deck<FooCard>)deck; // What should happen here?
FooCard foo = deck[0]; // or here?
So you need to ask yourself, "Why does the Card class care which deck owns it?" In most programs I'm aware of, there is no reason for a card to be aware of that detail. If there is a specific reason you want it to know something about its Deck, how much does it really need to know about that Deck? For example, is it possible that a Deck could hold cards from a variety of different Deck types? When you get cards out of the deck, do you really need to know that they will be exactly the same type of card as your current card?
Once you have answered questions like these, you'll be more prepared to decide whether OwningDeck should be a Deck<T> or maybe some covariant or contravariant interface that Deck<T> implements, or whether it even belongs on the Card class at all.

Accessing type of interface implementer in interface methods

public interface IMy<T>
{
T Implementer
{
get;
}
}
public class MyClass : IMy<MyClass>
{
...
}
But what I want is this:
public interface IMy
{
I Implementer
{
get;
}
}
Please accept that for some ca-razy reason I need an interface that defines a method that returns in the type of the implementer. No? Okay, suppose it's something like an XML explorer, so call my interface ITree for example.
Interfaces primarily help me organize (separate), but when I have one that requires I rely on a convention, it seems to defeat the purpose.

You want to be able to say "The interface should know who implements it, and automatically provide the implementer's type in its definition, to be inherited by the implementer"? Like
public interface IMy
{
ImplicitImplementerType SomeProperty
{
get;
}
}
I'm pretty sure that is impossible. Such a thing may be achieved in more dynamic languages though (I'm guessing here).

You cannot statically specify such a return type. In C# (and in the CLR) there is no "thistype". But you can find it out at runtime:
var thisType = this.GetType()
Since I don't think this is what you need, I have to inform you that it is not possible to do what you want.
Of course, the first code snippet of yours would work but you don't want that.

How to make 2 incompatible types, but with the same members, interchangeable?

Yesterday 2 of the guys on our team came to me with an uncommon problem. We are using a third-party component in one of our winforms applications. All the code has already been written against it. They then wanted to incorporate another third-party component, by the same vender, into our application. To their delight they found that the second component had the exact same public members as the first. But to their dismay, the 2 components have completely separate inheritance hierarchies, and implement no common interfaces. Makes you wonder... Well, makes me wonder.
An example of the problem:
Incompatible Types http://www.freeimagehosting.net/uploads/f9f6b862f1.png
public class ThirdPartyClass1
{
public string Name
{
get
{
return "ThirdPartyClass1";
}
}
public void DoThirdPartyStuff ()
{
Console.WriteLine ("ThirdPartyClass1 is doing its thing.");
}
}
public class ThirdPartyClass2
{
public string Name
{
get
{
return "ThirdPartyClass2";
}
}
public void DoThirdPartyStuff ()
{
Console.WriteLine ("ThirdPartyClass2 is doing its thing.");
}
}
Gladly they felt copying and pasting the code they wrote for the first component was not the correct answer. So they were thinking of assigning the component instant into an object reference and then modifying the code to do conditional casts after checking what type it was. But that is arguably even uglier than the copy and paste approach.
So they then asked me if I can write some reflection code to access the properties and call the methods off the two different object types since we know what they are, and they are exactly the same. But my first thought was that there goes the elegance. I figure there has to be a better, graceful solution to this problem.

My first question was, are the 2 third-party component classes sealed? They were not. At least we have that.
So, since they are not sealed, the problem is solvable in the following way:
Extract a common interface out of the coinciding members of the 2 third-party classes. I called it Icommon.
public interface ICommon
{
string Name
{
get;
}
void DoThirdPartyStuff ();
}
Then create 2 new classes; DerivedClass1 and DerivedClass2 that inherit from ThirdPartyClass1 and ThirdPartyClass2 respectively. These 2 new classes both implement the ICommon interface, but are otherwise completely empty.
public class DerivedClass1
: ThirdPartyClass1, ICommon
{
}
public class DerivedClass2
: ThirdPartyClass2, ICommon
{
}
Now, even though the derived classes are empty, the interface is satisfied by the base classes, which is where we extracted the interface from in the first place.
The resulting class diagram looks like this.
alt text http://www.freeimagehosting.net/uploads/988cadf318.png
So now, instead of what we previously had:
ThirdPartyClass1 c1 = new ThirdPartyClass1 ();
c1. DoThirdPartyStuff ();
We can now do:
ICommon common = new DerivedClass1 ();
common. DoThirdPartyStuff ();
And the same can be done with DerivedClass2.
The result is that all our existing code that referenced an instance of ThirdPartyClass1 can be left as is, by just swapping out the ThirdPartyClass1 reference for a ICommon reference. The ICommon reference could then be given an instance of DerivedClass1 or DerivedClass2, which of course in turn inherits from ThirdPartyClass1 and ThirdPartyClass2 respectively. And all just works.
I do not know if there is a specific name for this, but to me it looks like a variant of the adaptor pattern.
Perhaps we could have solve the problem with the dynamic types in C# 4.0, but that would have not had the benefit of compile-time checking.
I would be very interested to know if anybody else has another elegant way of solving this problem.

If you're using .Net 4 you can avoid having to do alot of this as the dynamic type can help with what you want. However if using .Net 2+ there is another (different way) of achieving this:
You can use a duck typing library like the one from Deft Flux to treat your third party classes as if they implemented an interface.
For example:
public interface ICommonInterface
{
string Name { get; }
void DoThirdPartyStuff();
}
//...in your code:
ThirdPartyClass1 classWeWishHadInterface = new ThirdPartyClass1()
ICommonInterface classWrappedAsInterface = DuckTyping.Cast<ICommonInterface>(classWeWishHadInterface);
classWrappedAsInterface.DoThirdPartyStuff();
This avoids having to build derived wrapper classes manually for all those classes - and will work as long as the class has the same members as the interface

What about some wrappers?
public class ThirdPartyClass1 {
public string Name {
get {
return "ThirdPartyClass1";
}
}
public void DoThirdPartyStuff() {
Console.WriteLine("ThirdPartyClass1 is doing its thing.");
}
}
public interface IThirdPartyClassWrapper {
public string Name { get; }
public void DoThirdPartyStuff();
}
public class ThirdPartyClassWrapper1 : IThirdPartyClassWrapper {
ThirdPartyClass1 _thirdParty;
public string Name {
get { return _thirdParty.Name; }
}
public void DoThirdPartyStuff() {
_thirdParty.DoThirdPartyStuff();
}
}
...and the same for ThirdPartyClass2, then you use the wrapper interface in all your methods.

Add an interface. You could add one wrapper (that implements the interface) for each of the 3rd parties.
Anyway, if you have the code of those 3rd parties, you could skip the wrapper thing and directly implement the interface. I'm quite sure you don't have the source, though.

Why cast to an interface?

In Jesse Liberty's Programming C# (p.142) he provides an example where he casts an object to an interface.
interface IStorable
{
...
}
public class Document : IStorable
{
...
}
...
IStorable isDoc = (IStorable) doc;
...
What is the point of this, particularly if the object's class implements the inteface anyway?
EDIT1: To clarify, I'm interested in the reason for the cast (if any), not the reason for implementing interfaces. Also, the book is his 2001 First Edition (based on C#1 so the example may not be germane for later versions of C#).
EDIT2: I added some context to the code

Because you want to restrict yourself to only methods provided by the interface. If you use the class, you run the risk of calling a method (inadvertently) that's not part of the interface.

There is only one reason when you actually need a cast: When doc is of a base type of an actual object that implements IStorable. Let me explain:
public class DocBase
{
public virtual void DoSomething()
{
}
}
public class Document : DocBase, IStorable
{
public override void DoSomething()
{
// Some implementation
base.DoSomething();
}
#region IStorable Members
public void Store()
{
// Implement this one aswell..
throw new NotImplementedException();
}
#endregion
}
public class Program
{
static void Main()
{
DocBase doc = new Document();
// Now you will need a cast to reach IStorable members
IStorable storable = (IStorable)doc;
}
}
public interface IStorable
{
void Store();
}

If the object implements the interface explicitly (public void IStorable.StoreThis(...)) that casting is the easiest way to actually reach the interface members.

I am not sure under what context the example was given in the book. But, you generally can type cast an object to interface to achieve multiple inheritance. I have given the example below.
public interface IFoo
{
void Display();
}
public interface IBar
{
void Display();
}
public class MyClass : IFoo, IBar
{
void IBar.Display()
{
Console.WriteLine("IBar implementation");
}
void IFoo.Display()
{
Console.WriteLine("IFoo implementation");
}
}
public static void Main()
{
MyClass c = new MyClass();
IBar b = c as IBar;
IFoo f = c as IFoo;
b.Display();
f.Display();
Console.ReadLine();
}
This would display
IBar implementation
IFoo implementation

It's pretty hard to tell without more of the context. If the variable doc is declared to be a type which implements the interface, then the cast is redundant.
Which version of the book are you reading? If it's "Programming C# 3.0" I'll have a look tonight when I'm at home.
EDIT: As we've seen in the answers so far, there are three potential questions here:
Why cast in the statement shown in the question? (Answer: you don't have to if doc is of an appropriate compile-time type)
Why is it ever appropriate to explicitly cast to an implemented interface or base class? (Answer: explicit interface implementation as shown in another answer, and also for the sake of picking a less specific overload when passing the cast value as an argument.)
Why use the interface at all? (Answer: working with the interface type means you're less susceptible to changes in the concrete type later on.)

The doc object might be of a type that implements members of IStorable explicitly, not adding them to the classes primary interface (i.e., they can only be called via the interface).
Actually "casting" (using the (T) syntax) does not make any sense since C# handles upcasts (cast to parent type) automatically (unlike F# for instance).

There are a lot of good answers here, but I don't really think they answer WHY you actually WANT to use the most restrictive interface possible.
The reasons do not involve your initial coding, they involve the next time you visit or refactor the code--or when someone else does it.
Let's say you want a button and are placing it on your screen. You are getting the button either passed in or from another function, like this:
Button x=otherObject.getVisibleThingy();
frame.add(x);
You happen to know that VisibleThingy is a button, it returns a button, so everything is cool here (no cast required).
Now, lets say that you refactor VisibleThingy to return a toggle button instead. You now have to refactor your method because you knew too much about the implementation.
Since you only NEED the methods in Component (a parent of both button and Toggle, which could have been an interface--same thing pretty much for our purposes), if you had written that first line like this:
Component x=(Component)otherObject.getVisibleThingy();
You wouldn't have had to refactor anything--it would have just worked.
This is a very simple case, but it can be much more complex.
So I guess the summary would be that an interface is a specific way to "View" your object--like looking at it through a filter...you can only see some parts. If you can restrict your view enough, the object can "Morph" behind your particular view and not effect anything in your current world--a very powerful trick of abstraction.

The best reason why you would cast to interfaces would be if you are writing code against objects and you don't know what concrete type they are and you don't want to.
If you know that you might come across an object that implements a specific interface you could then get the values out of the object without having to know the concrete class that this object is. Also, if you know that an object implements a given interface, that interface might define methods that you can execute to take certain actions on the object.
Here's a simple example:
public interface IText
{
string Text { get; }
}
public interface ISuperDooper
{
string WhyAmISuperDooper { get; }
}
public class Control
{
public int ID { get; set; }
}
public class TextControl : Control, IText
{
public string Text { get; set; }
}
public class AnotherTextControl : Control, IText
{
public string Text { get; set; }
}
public class SuperDooperControl : Control, ISuperDooper
{
public string WhyAmISuperDooper { get; set; }
}
public class TestProgram
{
static void Main(string[] args)
{
List<Control> controls = new List<Control>
{
new TextControl
{
ID = 1,
Text = "I'm a text control"
},
new AnotherTextControl
{
ID = 2,
Text = "I'm another text control"
},
new SuperDooperControl
{
ID = 3,
WhyAmISuperDooper = "Just Because"
}
};
DoSomething(controls);
}
static void DoSomething(List<Control> controls)
{
foreach(Control control in controls)
{
// write out the ID of the control
Console.WriteLine("ID: {0}", control.ID);
// if this control is a Text control, get the text value from it.
if (control is IText)
Console.WriteLine("Text: {0}", ((IText)control).Text);
// if this control is a SuperDooperControl control, get why
if (control is ISuperDooper)
Console.WriteLine("Text: {0}",
((ISuperDooper)control).WhyAmISuperDooper);
}
}
}
running this little program would give you the following output:
ID: 1
Text: I'm a text control
ID: 2
Text: I'm another text control
ID: 3
Text: Just Because
Notice that I didn't have to write any code in the DoSomething method that required me to know anything about all the objects I was working on being concrete object types. The only thing that I know is that I'm working on objects that are at least an instance of the Control class. I can then use the interface to find out what else they might have.
There's a million different reasons that you would take this approach with interfaces on your objects but it gives you a loose way to access your objects without having to know exactly what it is.
Think of all the credit cards in the world, every company makes their own, the interface is the same though, so every card reader can have a card swiped through it that follows the standard. Similar to the usage of interfaces.

As has been noted, the casting is superfluous and not necessary. However, it is a more explicit form of coding which would be useful to beginners in aiding their understanding.
In an introductory textbook, it is best to explicitly act, rather than let the compliler do things implicitly, which would be more confusing for beginners.
The "doc" is not of type "IStorable" so it would be confusing for beginners to see that it is being assigned to a isDoc. By explicitly casting, the author (of the book and of the code) is saying that a document can be casted to an IStorable object, but it is NOT THE SAME as an IStorable object.

The point is, the object (where did you get it?) may not implement the interface, in which case an exception is thrown which can be caught and dealt with. Of course you can use the "is" operator to check, and the "as" operator to cast instead of the C-style cast.

To allow for the most decoupling between pieces of code...
See the following article for more:
Interfaces

The main reason you would explicitly cast to an interface is if the members of the interface are implemented explicitly (i.e. with fully-qualified names in the form of InterfaceName.InterfaceMemberName). This is because when you fully-qualify them with the interface name, those members are not actually part of the implementing class's API. You can only get to them via casting to the interface.
Here's an example you can run as-is:
using System;
public interface ISomethingDoer {
void DoSomething();
}
public class ThingA : ISomethingDoer {
public void DoSomething(){
Console.WriteLine("ThingA did it!");
}
}
public class ThingB : ISomethingDoer {
// This is implemented explicitly by fully-qualifying it with the interface name
// Note no 'scope' here (e.g. public, etc.)
void ISomethingDoer.DoSomething(){
Console.WriteLine("ThingB did it!");
}
}
public static class Runner {
public static void Main(){
var a = new ThingA();
a.DoSomething(); // Prints 'ThingA did it!'
var b = new ThingB();
b.DoSomething(); // NOTE: THIS WILL NOT COMPILE!!!
var bSomethingDoer = (ISomethingDoer)b;
bSomethingDoer.DoSomething(); // Prints 'ThingB did it!'
}
}
HTH!