Why is there no Equals() or GetHashCode() in ArrayList?

[eramfelt]

Why doesnt ArrayList implements the Equals() or GetHashCode() methods?
How is the developer supposed to check if an array list is equal to
another array list? Is the developer supposed to iterate through the
array list to find out if they are equal or not? This should be handled
by the framework classes and I don't want to do any unnecessary
implementation (read copy-paste-problems).

If I look through the other classes in System.Collection I see that no
implements GetHashCode() or Equals(). If none of them do, then it must
be a design choice. But what design choice would that be? If anyone
have any explanation, please let me know.I really want to know.

There is an interface in System.Collections.Generic that is named
IEqualityComparer, which has two methods, Equals() and GetHashCode().
Why make an interface when every class implements those methods? And
why doesn't the default list/collection classes implement this
interface?

<flamebait-to-be-ignored>
If I look to Java, the ArrayList implements equals(), hashCode() as all
good objects should. They have also a good explanation on how it is
done here.
http://shorl.com/gogedrikekati
http://shorl.com/fygafrebrypeko
</flamebait-to-be-ignored>

Sorry that this post was double posted into the usenet aspnet group.

Regards
http://redsolo.blogspot.com

 

[Guest]

What would the arraylist return for GetHashCode, would it aggregate the
hascodes of your classes. As for equals, would you have it check to see if
all of the elements are equal, should they be equal in reference or in value,
and does that make the arraylist equal or is it equal if it points to the
same actual arraylist. The last one is the one the COR team chose and if you
want to have a difference meaning for equals then you should implement it.
In .NET 2, the list has the True for all predicate which you could pass an
anonymous method to to compare is to the first element, this would be like
the equals you are after but in a short bit of code.

Ciaran O'Donnell
http://wannabedeveloper.spaces.live.com

 

[eramfelt]

Ciaran O''Donnell skrev:

What would the arraylist return for GetHashCode, would it aggregate the
hascodes of your classes. As for equals, would you have it check to see if
all of the elements are equal, should they be equal in reference or in value,
and does that make the arraylist equal or is it equal if it points to the
same actual arraylist. The last one is the one the COR team chose and if you
want to have a difference meaning for equals then you should implement it.

If you check the two links my original post contained, you can read
another approach to this. They would aggregate the hashcodes of the
contained objects and return that. That way you can have two lists
generate the same hashcode if they contains "equal" objects. Which I
needed as I had two array lists at the server and client, and the
simplest way to check if they were equal would be by using the
GetHashCode().

For equality, it is very simple. The list should go through the
contained objects and check if they are "equal". Now it is up to the
contained class/objects to define "equality". The objects/classes can
define if they should be equal in value or in reference. That way the
framework gives the option to the developer to implement Equals() or
not. But unfortunately this decision is already made for the developer.

The default equals method is to check if the two object references are
the same. That is exactly similar to the "obj1 == obj2" statement. In
my view all good classes should implement Equals() (and therefore
GetHashCode()), so the user of a class can make a choice in either
check for reference equality OR value equality.

In .NET 2, the list has the True for all predicate which you could pass an
anonymous method to to compare is to the first element, this would be like
the equals you are after but in a short bit of code.

Yes, that would be less code. But it would be much less code if the
Collection classes implemented Equals(). Right?

It is a nice feature that developers can easily change the equality
meaning, but I havent done so in a real project yet.

//E

 

[Guest]

You can always subclass or wrap the .NET collections to add this yourself.
I'm sure MS had a reason not to include it and I think it was probably
because people would mistake the true function of the method and use it
wrong.
I have overridden equals in possibly every project I have worked on. It
enables so many things for you, especially working with business objects and
collections.

 

[eramfelt]

That is why I posted this question, to find out the reason to why they
choose not to implement it. But I still havent found a reason for it,
but it seems that they didn't want to implement it on collections,
since it is not a pure value type. But in my eyes ArrayList is just a
sequence of (potential) value types, and therefore it should implement
it.

But, if anyone knows. Please let me know.

You can always subclass or wrap the .NET collections to add this yourself.
I'm sure MS had a reason not to include it and I think it was probably
because people would mistake the true function of the method and use it
wrong.
I have overridden equals in possibly every project I have worked on. It
enables so many things for you, especially working with business objects and
collections.

Impl. Equals/GetHashCode() sure helps when Im unit testing.

//E