I'll demonstrate this premise with a fairly trivial example. An examination of the following function will reveal nothing obviously unsafe. Multiplying the two maximum possible function arguments that could be received (32767 * 32767) will never produce a result bigger than can be stored in the long variable that this function returns.
Private Function BonusCalc(ByVal niNumberOfWeeks As Integer, _ ByVal niWeeklyBonus As Integer) As Long BonusCalc = niNumberOfWeeks * niWeeklyBonus End Function
Now if you happened to be diligent enough to receive a weekly bonus of $1,000 over a period of 35 weeks…well, let's just say that this particular function wouldn't deliver your expected bonus! Although the function looks safe enough, Visual Basic's intermediate calculations behind the scenes cause trouble. When multiplying the two integers together, Visual Basic attempts to store the temporary result into another integer before assigning it to BonusCalc. This, of course, causes an immediate overflow error. What you have to do instead is give the Visual Basic compiler some assistance. The following revised statement works because Visual Basic realizes that we might be dealing with longs rather than just integers:
BonusCalc = niNumberOfWeeks * CLng(niWeeklyBonus)
Dealing with these sorts of language quirks is not easy. Programmers are often pushed for time, so they sometimes tend to avoid experimenting with a feature to see how it really works in detail. For the same reasons, reading the manuals or online help is often confined to a hasty glance just to confirm syntax. These are false economies. Even given the fact that sections of some manuals appear to have been written by Urdu swineherders on some very heavy medication, those pages still contain many pearls. When you use something in Visual Basic 6 for the first time, take a few minutes to read about its subtleties in the documentation and write a short program to experiment with its implementation. Use it in several different ways within a program, and twist it into funny shapes. Find out what it can and can't handle.
Some final thoughts Professional developers should understand the tools at their disposal at a detailed level. Learn from the manual how the tools should work, and then go beyond the manual and find out how they really work.
Yet More Schoolwork
Visual Basic 4 introduced the concept of object-oriented programming using the Basic language. Visual Basic 5 and 6 take this concept and elaborate on it in several ways. It is still possible to write Visual Basic 6 code that looks almost exactly like Visual Basic 3 code or that even resembles procedural COBOL code (if you are intent upon imitating a dinosaur). The modern emphasis, however, is on the use of relatively new ideas in Basic, such as abstraction and encapsulation, which aim to make applications easier to develop, understand, and maintain. Any Visual Basic developer unfamiliar with these ideas first has to learn what they are and why they are useful and then has to understand all the quirks of their implementation in Visual Basic 6. The learning curve is not trivial. For example, understanding how the Implements statement produces a virtual class that is Visual Basic 6's way of implementing polymorphism (and inheritance if you're a little sneaky) can require some structural remodeling of one's thought processes. This is heavy-duty object-oriented programming in the 1990s style. Trying to use it in a production environment without a clear understanding is a prime cause of new and unexpected bugs.
Developers faced with radically new concepts usually go through up to four stages of enlightenment. The first stage has to do with reading and absorbing the theory behind the concept. The second stage includes working with either code examples or actual programs written by other people that implement the new concept. The third stage involves using the new concept in their own code. Only at this point do programmers become fully aware of the subtleties involved and understand how not to write their code. The final stage of enlightenment arrives when the programmer learns how to implement the concept correctly, leaving no holes for the bugs to crawl through.
Some final thoughts Developers should take all the time necessary to reach the third and fourth stages of enlightenment when learning new programming concepts or methodologies. Only then should they be allowed to implement these new ideas in production systems.