[Ironruby-core] object equality

Davy Brion ralinx at davybrion.com
Thu Sep 9 07:44:39 EDT 2010


by default, == does a reference check in C#, unless you override it to do a
value based check (which you typically implement in Equals)

in C#, if you want == and != to work properly you need to implement them
both.  In ruby, you obviously can't implement !=, but i had (naievely
perhaps) expected that IronRuby would preserve the Ruby behavior when
calling == on an object which implements it.  I _think_ that would be the
best way to handle this, though there might be very valid reasons as to why
this isn't the case at the moment.

defining an Equals method on the ruby class which delegates to == works, but
it is somewhat weird since most people use the == and != operators to check
for equality.

perhaps i'm better off getting rid of the == implementation and solely
providing an Equals implementation

On Thu, Sep 9, 2010 at 1:10 PM, William Green <will at hotgazpacho.org> wrote:

> Then it would appear that in C#, using the != operator on two instances of
> Ruby objects does not call the == method on the first Ruby object and invert
> the result.
>
> Can you switch to using equals as a work-around?
>
> Not sure of the semantics around == vs .Equals in C#, but I know there is a
> semantic difference between == and eql? in Ruby.
>
> --
> Will Green
> http://hotgazpacho.org/
>
>
>
> On Sep 9, 2010, at 6:52 AM, Davy Brion <ralinx at davybrion.com> wrote:
>
> the problem isn't with checking wether 2 objects are equal (though you
> indeed need to define an Equals method on your ruby object if you want the
> comparison to work with a direct call to .Equals... doing == in C#
> definitely uses the == method of your ruby object) but it is with the !=
> check.  In ruby, using != calls == and inverts the result of that.  Doing !=
> in C# on a ruby object doesn't seem to do the same thing.
>
> I'm also not entirely sure how it _should_ be... but as far as i can tell,
> right now, i can't get equality checks working properly with ruby objects.
>
> if a == b is true, then a != b should always be false
> if a.Equals(b) is true, then !a.Equals(b) should always be false
>
> if there's another way to get this behavior working in C# for ruby objects,
> i'd love to hear about it since it's pretty important for something i'm
> trying to do :)
>
> On Thu, Sep 9, 2010 at 12:20 PM, William Green < <will at hotgazpacho.org>
> will at hotgazpacho.org> wrote:
>
>> Testing for object equality in C# is different than it is in Ruby. In
>> C#, you need to override both Object.Equals and Object.GetHashCode (I
>> forget which is used when, but I do recall that the compiler complains
>> if you override one and not the other). So, when you bring your Ruby
>> object into C# and compare them, C# doesn't see an override for Equals
>> on your object, and thus uses Object.Equals (which is often what you
>> don't want). Try defining an equals method on your Ruby object, or
>> alias it to ==.
>>
>> I suspect that the == method on your Ruby object does not map to
>> Equals when you bring into C#. And I'm not sure that it should.
>>
>> Thoughts?
>>
>> --
>> Will Green
>>  <http://hotgazpacho.org/>http://hotgazpacho.org/
>>
>>
>>
>> On Sep 9, 2010, at 4:33 AM, Davy Brion < <ralinx at davybrion.com>
>> ralinx at davybrion.com> wrote:
>>
>> > If i have the following class in ruby:
>> >
>> > class TestClass
>> >   def initialize(value)
>> >     @value = value
>> >   end
>> >
>> >   def ==(other)
>> >     return false if other.nil?
>> >     self.value == other.value
>> >   end
>> >
>> >   protected
>> >
>> >   def value
>> >     @value
>> >   end
>> > end
>> >
>> > test1 = TestClass.new(5)
>> > test2 = TestClass.new(5)
>> > p test1 == test2
>> > p test1 != test2
>> >
>> > the output is:
>> > true
>> > false
>> >
>> > if i do this in .NET:
>> >
>> > dynamic test1 = ruby.TestClass. at new(5);
>> > dynamic test2 = ruby.TestClass. at new(5);
>> >
>> > var equals = test1 == test2;
>> > var differs = test1 != test2;
>> >
>> > both equals and differs are true
>> >
>> > i'm going to create an issue about this, but i do need to get this
>> working... is there a temporary workaround that i can use for now?
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