Database Design – Should False Be Stored as Null in a Boolean Field?

The question is not so much whether you should check for null or let the runtime throw an exception; it is how you should respond to such an unexpected situation.

Your options, then, are:

• Throw a generic exception (NullReferenceException) and let it bubble up; if you don't do the null check yourself, this is what happens automatically.
• Throw a custom exception that describes the problem on a higher level; this can be achieved either by throwing in the null check, or by catching a NullReferenceException and throwing the more specific exception.
• Recover by substituting a suitable default value.

There is no general rule which one is the best solution. Personally, I would say:

• The generic exception is best if the error condition would be a sign of a serious bug in your code, that is, the value that is null should never be allowed to get there in the first place. Such an exception may then bubble all the way up into whatever error logging you have set up, so that someone gets notified and fixes the bug.
• The default value solution is good if providing semi-useful output is more important than correctness, such as a web browser that accepts technically incorrect HTML and makes a best effort to render it sensibly.
• Otherwise, I'd go with the specific exception and handle it somewhere suitable.

I would put the null checks in your mapping code, where you build your object from the result set. That puts the checking in one place, and won't allow your code to get halfway through processing a record before hitting an error. Depending on how your application flow works, you might want to perform the mapping of all the results as a pre-processing step instead of mapping and processing each record one at a time.

If you're using an ORM then you'll have to perform all your null checks before processing each record. I'd recommend a recordIsValid(recordData)-type method, that way you can (again) keep all the null-checking and other validation logic in one place. I definitely wouldn't intermingle the null checks with the rest of your processing logic.