The Collapse of Boolean Logic

What will this code write to the console?

public void SomeMethod()
{
  SqlInt32 x = 1;

  if(x == SqlInt32.Null)
  {
    Console.WriteLine("Null.");
  }
  else if(x != SqlInt32.Null)
  {
    Console.WriteLine("Not Null.");
  }
  else
  {
    Console.WriteLine("Neither? WTF!?");
  }
}

If you said “Neither? WTF!?” then you’d be correct.

Doesn’t that seem odd since x is either SqlInt32.Null or it isn’t? Is this a case of fuzzy logic? Well not exactly. This is one of those gotchas with operator overloading. The == operator is overloaded by SqlInt32 to return a SqlBoolean instead of a bool.

This caused me a few minutes of pain this week as I was stepping through code like this and examining the values in the debugger and I thought perhaps someone had slipped me a very strong hallucinogen because there I was with a value that was not null, but was null at the same time. It was one of those mind warping “This sentence is not true” moments.

As typical, I was thinking about rebooting when on a whim I decided to use intermediate variables and realized that the == was returning something neither true nor false. Honestly, I think this is a very poor and unnecessary use of operator overloading (correct me if I’m wrong) because it hides a real gotcha underneath a very common paradigm. If you overload the == operator, you should most definitely return a bool.

In this case, it’s easy enough to fix. I should have been checking the IsNull property anyways like so:

public void SomeMethod()
{
  SqlInt32 x = 1;

   if(x.IsNull)
  {
     Console.WriteLine("Null.");
  }
  else
  {
     Console.WriteLine("Not Null.");
  }
}

This post by Cyrus motivated me to write about this.