A Whole Lot Of Nothing

Occasionally I get questions from people who are confused over the semantics of data that are not even there. Usually they’ve written code something like

If Blah = Nothing Then

or

If Blah = Empty Then

or

If Blah = Null Then

all three of which almost certainly do not correctly express the actual intention of the
programmer. Why does VBScript have Null, Nothing and Empty, and what are the differences between them?

Let’s start with Empty. When you declare a variable in C, the variable’s value before the first assignment is undefined:

int index;
printf("%d", index); /* could print any integer */

In C, the declaration reserves space for the variable, but does not clear the contents of that space. After all, why would it need to? You’re just going to initialize it to some value yourself, right? Why should the compiler waste time by initializing it only to have that initialization overwritten?

That might seem like a sensible attitude if you are one of those people who prefers that your program be twenty or even thirty nanoseconds faster in exchange for causing any accidental use of uninitialized memory to make your program’s behaviour completely random.

The designers of VB knew that their users were not hard core bit twiddling performance wonks, but rather line-of-business developers who prefer a predictable programming environment. Thus, VB initializes variables as they are declared and eats a few processor cycles here and there. When you declare an integer in VB, it’s initialized to zero, strings are initialized to empty strings, and so on.

But what about variants? Should an uninitialized variant be initialized to zero? That seems bogus; why should an uninitialized variant automatically become a number?

Really what we need is a special of-no-particular-type “I’m an uninitialized variant” value, and that’s Empty. And since in VBScript, all variables are variants, all variables are initialized to Empty.

What if in VB you compare an uninitialized variant to an uninitialized integer? It seems sensible that the comparison would return True, and it does. Empty compares as equal to 0 and the empty string, which might cause false positives in our example above. If you need to detect whether a variable actually is an empty variant and not a string or a number, you can use IsEmpty. (Alternatively, you could use TypeName or VarType, but I prefer IsEmpty.)

Nothing is similar to Empty but subtly different. Empty says “I am an uninitialized variant”; Nothing says “I am an object reference that refers to no object”. Since the equality operator on objects checks for equality on the default property of an object, any attempt to say

If Blah = Nothing Then

is doomed to failure — Nothing does not have a default property, so this will produce a run-time error. To check to see if an object reference is invalid, use

If Blah Is Nothing Then

Null is weirder still. The semantics of Null are very poorly understood, particularly amongst people who have little experience with relational databases. Empty says “I’m an uninitialized variant”, Nothing says “I’m an invalid object” and Null says “I represent a value which is not known.”

Let me give an example. Suppose you have a database of sales reports, and you ask the database “what was the total of all sales in August?” but one of the sales staff has not reported their sales for August yet. What’s the correct answer? You could design the database to ignore the fact that data is missing and give the sum of the known sales, but that would be answering a different question. The question was not “what was the total of all known sales in August, excluding any missing data?” The question was “what was the total of all sales in August?” The answer to that question is “I don’t know — there is data missing”, so the database returns Null.

What happens when you say

If Blah = Null Then

?

Let’s try printing out the value of the comparison and see:

Sales = 123
WScript.Echo Sales = Null

You get not True, not False, but Null! Why’s that? Well, think about the semantics
of it. You’re saying “is the unknown quantity equal to 123?” The answer to that is not “yes”, it’s not “no”, it’s “I don’t know what the unknown quantity is, so, uh, maybe?”

Nulls propagate themselves. Any time you numerically manipulate a Null,
you get a Null right back. Any sum containing an unknown addend has an unknown sum, obviously! The correct way to check for Null is much as you’d do for Empty:
use IsNull (or TypeName or VarType.)

The sharp-eyed among you will have noticed that I never actually answered the question. What does happens when you say

If Blah = Null Then

does VBScript run the consequence block or the alternative (“else”) block? Obviously it has to do one of the two. When it comes right down to it, VBScript will assume falsity in this situation.

The way JScript and JScript .NET handle nulls is a little bit weird; I’ll talk about that in my next entry.


Commentary from 2019:

Most of the comments of this posting were a discussion with a reader who used to use the information in this blog post as interview questions; I pushed back on that, as I want interview questions to elicit signal on skills, not specific trivial knowledge. Sure, you can tell to what level a developer actually knows their tools by that sort of probing, but I am much more interested in a developer’s ability to analyze problems, find solutions, and write solid code. The original reader was mostly looking to interview for contract positions, where there was an expectation of expertise that could lead to immediate productivity; I tend to interview not for immediate productivity, but ability to learn.

Peter Torr pointed out that I had missed one on my list of “missing data” types. VBScript represents a call to a function with an omitted optional argument as a variant of type VT_ERROR with value DISP_E_PARAMNOTFOUND.

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