Inconsistency in Equals and GetHashCode methods

Question

After reading this question Why do "int" and "sbyte" GetHashCode functions generate different values? I wanted to dig further and found following behavior:

sbyte i = 1;            
int j = 1;
object.Equals(i, j) //false (1)
object.Equals(j, i) //false (2) 
i.Equals(j) //false (3)
j.Equals(i) //true (4)
i == j //true (5)
j == i //true (6)
i.GetHashCode() == j.GetHashCode() //false (7)

Difference between (3) and (4) breaks the requirement that Equals should be symmetric.
Difference between (2) and (4) is not coherent with MSDN specification that says:

If the two objects do not represent the same object reference and neither is null, it calls objA.Equals(objB) and returns the result. This means that if objA overrides the Object.Equals(Object) method, this override is called.
Difference between (3) and (5) means that operator == returns true, however objects are not equal in terms of Equals.
Difference between (4), (5), (6) and (7) means that two objects are equal in terms of operator == and Equals, however they have different hash codes.

I'm very interested if anyone can explain why such in my opinion inconsistent behaviour is observed in rather fundamental .NET types.

That's was what I think when see that. Very thanks for see the core of the problem empi — Eric Javier Hernandez Saura
– Eric Javier Hernandez Saura, Commented Sep 19, 2012 at 20:47
All those 'inconsistencies' follow from the implicit conversion from sbyte to int. — Henk Holterman
– Henk Holterman, Commented Sep 19, 2012 at 20:55
@pst a.Equals((object)b) certainly implies a.GetHashCode()==b.GetHashCode(). And I'd call any type for which == behaves differently from Equals broken, even if .net doesn't strictly require this. (disregarding implicit conversions, and assuming runtime type and static type are identical) — CodesInChaos
– CodesInChaos, Commented Sep 19, 2012 at 20:57

CodesInChaos · Accepted Answer · 2012-09-19 20:51:23Z

15

Your problem is that you missed the implicit conversion in i.Equals(j). It goes to the overload int.Equals(int). Here you're comparing i and (int)j, which are the same thing. The same implicit conversion happens for ==.

The other comparisons work on an int and a sbyte, which by definition are different. j.Equals(i) goes to the overload int.Equals(object), because the argument isn't implicitly convertible to sbyte.

Equals is symmetric for them, but your calling code isn't. If you suppress the implicit conversion with i.Equals((object)j), it'll return false, showing that Equals is indeed symmetric.

edited Sep 19, 2012 at 20:51

answered Sep 19, 2012 at 20:45

CodesInChaos

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