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 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 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