C# 'is' type check on struct - odd .NET 4.0 x86 optimization behavior

Question

Update: I have filed a bug report with Microsoft Connect, please vote for it!

Update 2: Microsoft have marked the bug report as fixed

Answer 1

Other than a few nops, reflector indictes the only differences are in the IsGuid method:

DEBUG:

.method public hidebysig static bool IsGuid(object item) cil managed
{
    .maxstack 2
    .locals init (
        [0] bool CS$1$0000)
    L_0000: nop 
    L_0001: ldarg.0 
    L_0002: isinst [mscorlib]System.Guid
    L_0007: ldnull 
    L_0008: cgt.un 
    L_000a: stloc.0 
    L_000b: br.s L_000d
    L_000d: ldloc.0 
    L_000e: ret 
}

RELEASE:

.method public hidebysig static bool IsGuid(object item) cil managed
{
    .maxstack 8
    L_0000: ldarg.0 
    L_0001: isinst [mscorlib]System.Guid
    L_0006: ldnull 
    L_0007: cgt.un 
    L_0009: ret 
}

I'm not fluent enough to explain these differences but this is a community wiki answer so maybe somebody else can enlighten us?

Changing the method to make it generic works around this (possible bug?)

public static bool IsGuid(T item)
{
    return item is Guid;
}

As does forcing it into a local varible (but it must be used in the method to prevent the optimizations kicking in):

public static bool IsGuid(object item)
{
    bool a = item is Guid;
    a.ToString();
    return a;
}