Unexpected behavior in c# generic method on .Equals

Question

Why does the Equals method return a different result from within the generic method? I think that there\'s some automatic boxing here that I don\'t understand.

Here\'s a

Answer 1

FWIW, on mono 2.8+ both return values are False, outputting

False
False

Amazingly, csc.exe from VS2010 produces different results, indeed, outputting:

False
True

Even more interestingly, the problem appears not with the generated IL code, but with the Framework implementation/JIT engine;

• executing the MS-compiled image with the Mono VM results in False/False, like the mono compiled version
• executing the Mono-compiled image with the MS VM results in False/True, like the MS compiled version

---

For your interest, here are the disassemblies of Microsoft's CSC.exe compiler (csc.exe /optimize+ test.cs):

.method private static hidebysig 
       default bool Compare<T> (!!T x, !!T y)  cil managed 
{
    // Method begins at RVA 0x2087
// Code size 30 (0x1e)
.maxstack 8
IL_0000:  ldarg.0 
IL_0001:  box !!0
IL_0006:  brfalse.s IL_001c

IL_0008:  ldarga.s 0
IL_000a:  ldarg.1 
IL_000b:  box !!0
IL_0010:  constrained. !!0
IL_0016:  callvirt instance bool object::Equals(object)
IL_001b:  ret 
IL_001c:  ldc.i4.0 
IL_001d:  ret 
} // end of method Program::Compare

and Mono's gmcs.exe compiler (dmcs -optimize+ test.cs):

.method private static hidebysig 
       default bool Compare<T> (!!T x, !!T y)  cil managed 
{
    // Method begins at RVA 0x212c
// Code size 33 (0x21)
.maxstack 4
IL_0000:  ldarg.0 
IL_0001:  box !!0
IL_0006:  brfalse IL_001f

IL_000b:  ldarga.s 0
IL_000d:  ldarg.1 
IL_000e:  box !!0
IL_0013:  constrained. !!0
IL_0019:  callvirt instance bool object::Equals(object)
IL_001e:  ret 
IL_001f:  ldc.i4.0 
IL_0020:  ret 
} // end of method Program::Compare

Answer 2

TimezoneInfo defines it's own overload of Equals(TimeZoneInfo). In the Compare method, the object equals is used (it's a virtual method call of Object.Equals), whereas in Console.WriteLine(tzOne.Equals(tzTwo)) the overloaded (new) TimeZoneInfo.Equals method is called.

TimeZoneInfo obviously hasn't overridden the Object.Equals method correctly...

Answer 3

TimeZoneInfo does not override the Object Equals method, so it calls the default Object Equals, which apparently does not work as expected. I would consider this a bug in TimeZoneInfo. This should work:

private static Boolean Compare<T>(T x, T y)
        where T: IEquatable<T>
{
    if (x != null)
    {
        return x.Equals(y);
    }
    return false;
}

The above will cause it to call Equals<T>, which is the method you were calling above (it implicitly preferred the generic call because it was more specific to the parameter type than the Object Equals; inside the generic method, however, it had no way to be sure that such a generic Equals existed, since there was no constraint guaranteeing this).