Why check this != null?

Occasionally I like to spend some time looking at the .NET code just to开发者_C百科 see how things are implemented behind the scenes. I stumbled upon this gem while looking at the String.Equals method via Reflector.

C#

[ReliabilityContract(Consistency.WillNotCorruptState, Cer.MayFail)]
public override bool Equals(object obj)
{
    string strB = obj as string;
    if ((strB == null) && (this != null))
    {
        return false;
    }
    return EqualsHelper(this, strB);
}

IL

.method public hidebysig virtual instance bool Equals(object obj) cil managed
{
    .custom instance void System.Runtime.ConstrainedExecution.ReliabilityContractAttribute::.ctor(valuetype System.Runtime.ConstrainedExecution.Consistency, valuetype System.Runtime.ConstrainedExecution.Cer) = { int32(3) int32(1) }
    .maxstack 2
    .locals init (
        [0] string str)
    L_0000: ldarg.1 
    L_0001: isinst string
    L_0006: stloc.0 
    L_0007: ldloc.0 
    L_0008: brtrue.s L_000f
    L_000a: ldarg.0 
    L_000b: brfalse.s L_000f
    L_000d: ldc.i4.0 
    L_000e: ret 
    L_000f: ldarg.0 
    L_0010: ldloc.0 
    L_0011: call bool System.String::EqualsHelper(string, string)
    L_0016: ret 
}

What is the reasoning for checking this against null? I have to assume there is purpose otherwise this probably would have been caught and removed by now.

---

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.

---

The reason why is that it is indeed possible for this to be null. There are 2 IL op codes which can be used to invoke a function: call and callvirt. The callvirt function causes the CLR to perform a null check when invoking the method. The call instruction does not and hence allows for a method to be entered with this being null.

Sound scary? Indeed it is a bit. However most compilers ensure this doesn't ever happen. The .call instruction is only ever outputted when null is not a possibility (I'm pretty sure that C# always uses callvirt).

This isn't true for all languages though and for reasons I don't exactly know the BCL team chose to further harden the System.String class in this instance.

Another case where this can popup is in reverse pinvoke calls.

---

The short answer is that languages like C# force you to create an instance of this class before calling the method, but the Framework itself does not. There are two¹ different ways in CIL to call a function: call and callvirt.... Generally speaking, C# will always emit callvirt, which requires this to not be null. But other languages (C++/CLI comes to mind) could emit call, which doesn't have that expectation.

(¹okay, it's more like five if you count calli, newobj etc, but let's keep it simple)

---

The source code has this comment:

this is necessary to guard against reverse-pinvokes and other callers who do not use the callvirt instruction

---

Let's see... this is the first string you're comparing. obj is the second object. So it looks like it's an optimization of sorts. It's first casting obj to a string type. And if that fails, then strB is null. And if strB is null while this isn't, then they're definitely not equal and the EqualsHelper function can be skipped.

That will save a function call. Beyond that, perhaps a better understanding of the EqualsHelper function might shed some light on why this optimization is needed.

EDIT:

Ah, so the EqualsHelper function is accepting a (string, string) as parameters. If strB is null, then that essentially means that it was either a null object to begin with, or it couldn't successfully be cast into a string. If the reason for strB being null is that the object was a different type that couldn't be converted to a string then you wouldn't want to call EqualsHelper with essentially two null values (that'll return true). The Equals function should return false in this case. So this if statement is more than an optimization, it actually ensures proper functionality as well.

---

If the argument (obj) does not cast to a string then strB will be null and the result should be false. Example:

    int[] list = {1,2,3};
    Console.WriteLine("a string".Equals(list));

writes false.

Remember that string.Equals() method is called for any argument type, not only for other strings.