dotNET Rant [modified]

[TheGreatAndPowerfulOz]

ok, this is not a programming question. It's a rant! given,

object one = 0;
object two = 0;
bool same = one == two;

what would you expect the value of same to be? WRONG! it's false! Whoever thought that was a valid result, is cracked!:mad::mad::mad::mad::mad: [edit] so, after going home and resting my brain a bit. it seems as though i'm the one that was cracked. thanks for the refresher course everyone. it is of course doing a reference comparison. which is correct. you all know how it is when you struggle with something and get too close to the trees to see the forest. anyway thanks to everyone for being your normally brutally honest selves. cheers. :-D [/edit]

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modified on Friday, May 7, 2010 1:08 AM

[Rama Krishna Vavilala]

ahmed zahmed wrote:

what would you expect the value of same to be?

It has to be false. What else can it be? I am pretty sure lot of other programmers here would have answered false too. :confused:

[Judah Gabriel Himango]

A subtlety of implicit boxing. What you did was create 2 new objects on the heap, each one pointing to an integer, 0. Then, you compared equality on the objects, not on the integers. You expected them to be equal because they're pointing to integers that happen to be equal, but you were comparing the objects, not the integers. To compare equality on the integers,

bool same = (int)one == (int)two;

Religiously blogging on the intarwebs since the early 21st century: Kineti L'Tziyon
Judah Himango

[Rama Krishna Vavilala]

Judah Himango wrote:

What you did was create 2 new objects on the heap, each one pointing to an integer, 0.

… and did a reference comparison.

[Luc Pattyn]

.NET is irrelevant here, we all know one and two are not the same. :)

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[TheGreatAndPowerfulOz]

Rama Krishna Vavilala wrote:

It has to be false. What else can it be?

Why? One would expect that the comparators of the boxed values would be used. so, the result would be true, as I initially expected. I understand what's happening, but I don't understand why the dotNETtors decided this was correct.

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[TheGreatAndPowerfulOz]

but in code that is comparing boxed values, you don't know what the boxed types are so it's not as simple as casting to a known type.

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[TheGreatAndPowerfulOz]

:laugh: :laugh: :laugh: ;P ;P :((

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[Rama Krishna Vavilala]

== is ref comparison so I would not expect the references to be equal. The right comparison would have been to use Equals.

[TheGreatAndPowerfulOz]

== calls Equals. Use Reflector.

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[Judah Gabriel Himango]

Code that compares boxed value types has to know about the value it's trying to compare, otherwise it isn't a very smart boxed value type comparer. Generics and EqualityComparer might help you here.

Religiously blogging on the intarwebs since the early 21st century: Kineti L'Tziyon
Judah Himango

[TheGreatAndPowerfulOz]

The context is comparing DbParameter's (parameters to a PreparedCommand) to know whether the result is cached or not. So it's not as simple as it may seem. In any case, seems to me that since only primitives get boxed, then that condition should be checked in the object.Equals code. Thanks for the suggestion.

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[Lost User]

Let's try

object x = 0;
object y = 0;
Console.WriteLine(x == y);

Result:

.locals init (
    \[0\] object x,
    \[1\] object y)
L\_0000: nop 
L\_0001: ldc.i4.0 
L\_0002: box int32
L\_0007: stloc.0    // x is boxed int
L\_0008: ldc.i4.0 
L\_0009: box int32
L\_000e: stloc.1    // y is boxed int
L\_000f: ldloc.0 
L\_0010: ldloc.1 
L\_0011: ceq        // comparison does NOT call Equals
L\_0013: call void \[mscorlib\]System.Console::WriteLine(bool)
L\_0018: nop 
L\_0019: br.s L\_001b
L\_001b: ret

[TheGreatAndPowerfulOz]

ok, an MSIL lawyer! perhaps, it got optimized away. In any case, the result is false.

Fight Big Government:
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[Rama Krishna Vavilala]

ahmed zahmed wrote:

== calls Equals

No. It calls Equals only when some class has overloaded the == operator (aka string). For objects == always means reference comparison. Also if it called Equals, you would not have had the problem in the first place. Because, one.Equals(two) will return true in your application. [Edit] Thanks for the one vote [/Edit]

[PIEBALDconsult]

Nooo... that's correct. Otherwise, what would you do with this:

int one = 0 ;
int two = 0 ;

bool same = (object) one == (object) two ;

Shouldn't this perform the same reference comparison of your code? (Man, you miss one closing quote... :-O )

modified on Thursday, May 6, 2010 7:38 PM

[TheGreatAndPowerfulOz]

eh? why are you showing me javascript?

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[Lost User]

ahmed zahmed wrote:

perhaps, it got optimized away.

I very much doubt it. The C# compiler only seems to do trivial constant folding (without using commutativity etc) and some limited dead code elimination (after an unconditional return etc) The JIT compiler does the rest (which is not a lot, either) If it changes the result it is not an "optimization" but a bug. And, this was a Debug build, as can easily be seen. Here is the same code compiled in Release mode.

.locals init (
    \[0\] object x,
    \[1\] object y)
L\_0000: ldc.i4.0 
L\_0001: box int32
L\_0006: stloc.0 
L\_0007: ldc.i4.0 
L\_0008: box int32
L\_000d: stloc.1 
L\_000e: ldloc.0 
L\_000f: ldloc.1 
L\_0010: ceq 
L\_0012: call void \[mscorlib\]System.Console::WriteLine(bool)
L\_0017: ret

[TheGreatAndPowerfulOz]

Whatever, the point is, it didn't do as, at least, *I* expected. Perhaps its a compiler optimization that it's able to do from context. Try this:

bool compare(object a, object b)
{
return a == b;
}

bool result = compare(0, 0);

I'm not sure if the actual result or my expectation is correct. In any case, I wasted a lot of time on this because staring at the code it sure looked like it should "work."

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[Lost User]

Hax edit: that was about what that post said when it was still breaking the forum.

modified on Friday, May 7, 2010 7:48 AM