Why check this != null?

I assume you were looking at the .NET 3.5 implementation? I believe the .NET 4 implementation is slightly different.

However, I have a sneaking suspicion that this is because it’s possible to call even virtual instance methods non-virtually on a null reference. Possible in IL, that is. I’ll see if I can produce some IL which would call null.Equals(null).

EDIT: Okay, here’s some interesting code:

.method private hidebysig static void  Main() cil managed
{
  .entrypoint
  // Code size       17 (0x11)
  .maxstack  2
  .locals init (string V_0)
  IL_0000:  nop
  IL_0001:  ldnull
  IL_0002:  stloc.0
  IL_0003:  ldloc.0
  IL_0004:  ldnull
  IL_0005:  call instance bool [mscorlib]System.String::Equals(string)
  IL_000a:  call void [mscorlib]System.Console::WriteLine(bool)
  IL_000f:  nop
  IL_0010:  ret
} // end of method Test::Main

I got this by compiling the following C# code:

using System;

class Test
{
    static void Main()
    {
        string x = null;
        Console.WriteLine(x.Equals(null));

    }
}

… and then disassembling with ildasm and editing. Note this line:

IL_0005:  call instance bool [mscorlib]System.String::Equals(string)

Originally, that was callvirt instead of call.

So, what happens when we reassemble it? Well, with .NET 4.0 we get this:

Unhandled Exception: System.NullReferenceException: Object
reference not set to an instance of an object.
    at Test.Main()

Hmm. What about with .NET 2.0?

Unhandled Exception: System.NullReferenceException: Object reference 
not set to an instance of an object.
   at System.String.EqualsHelper(String strA, String strB)
   at Test.Main()

Now that’s more interesting… we’ve clearly managed to get into EqualsHelper, which we wouldn’t have normally expected.

Enough of string… let’s try to implement reference equality ourselves, and see whether we can get null.Equals(null) to return true:

using System;

class Test
{
    static void Main()
    {
        Test x = null;
        Console.WriteLine(x.Equals(null));
    }

    public override int GetHashCode()
    {
        return base.GetHashCode();
    }

    public override bool Equals(object other)
    {
        return other == this;
    }
}

Same procedure as before – disassemble, change callvirt to call, reassemble, and watch it print true…

Note that although another answers references this C++ question, we’re being even more devious here… because we’re calling a virtual method non-virtually. Normally even the C++/CLI compiler will use callvirt for a virtual method. In other words, I think in this particular case, the only way for this to be null is to write the IL by hand.

---

EDIT: I’ve just noticed something… I wasn’t actually calling the right method in either of our little sample programs. Here’s the call in the first case:

IL_0005:  call instance bool [mscorlib]System.String::Equals(string)

here’s the call in the second:

IL_0005:  call instance bool [mscorlib]System.Object::Equals(object)

In the first case, I meant to call System.String::Equals(object), and in the second, I meant to call Test::Equals(object). From this we can see three things:

• You need to be careful with overloading.
• The C# compiler emits calls to the declarer of the virtual method – not the most specific override of the virtual method. IIRC, VB works the opposite way
• object.Equals(object) is happy to compare a null “this” reference

If you add a bit of console output to the C# override, you can see the difference – it won’t be called unless you change the IL to call it explicitly, like this:

IL_0005:  call   instance bool Test::Equals(object)

So, there we are. Fun and abuse of instance methods on null references.

If you’ve made it this far, you might also like to look at my blog post about how value types can declare parameterless constructors… in IL.