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

Interestingly enough this works correctly:

using System;
public class Program
{
    public static bool IsGuid(object item)
    {
        return item is Guid;
    }

    public static void Main()
    {
        Guid s = Guid.NewGuid();
        Console.Write(IsGuid(s));
    }
}

Here is the difference in the il:

.method public hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       23 (0x17)
  .maxstack  1
  .locals init (valuetype [mscorlib]System.Guid V_0)
  IL_0000:  call       valuetype [mscorlib]System.Guid [mscorlib]System.Guid::NewGuid()
  IL_0005:  stloc.0
  IL_0006:  ldloc.0
  IL_0007:  box        [mscorlib]System.Guid
  IL_000c:  call       bool Program::IsGuid(object)
  IL_0011:  call       void [mscorlib]System.Console::Write(bool)
  IL_0016:  ret
} // end of method Program::Main