What is this?

Recently I’ve been updating one of our older utilities to .NET 4.  A few days ago I stumbled across this line of C#:

if(this == null) return null;

I was dumbfounded.  When would that ever evaluate to true?  Worse yet, why was it repeated in two other places?

Out of curiosity (read: late night boredom) I did some research to see if there’s ever a case where the condition would be met and found a good discussion over on Stack Overflow.  There apparently are a few cases where this == null could actually be true:

  1. Overload the == operator to explicitly return true when comparing to null.
  2. Pass this to a base constructor as part of a closure.

Neither of these cases applied to this code.  We weren’t overloading the == operator and we certainly weren’t using it in a closure let alone a closure being passed to a base constructor.  The second case has apparently been fixed for .NET 4 so it definitely wouldn’t apply with the changes I was making.

As part of the Stack Overflow discussion Eric Lippert provided an interesting comment about why the C# compiler doesn’t complain about checking for this == null.   He basically says that the compiler doesn’t complain because they didn’t think about it because it’s obviously wrong.  So for those wondering, yes, I eliminated all three instances of this code from the utility.