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Error Handling the VB .NET Way: Living with Exceptions UP TO NOW , WE HAVE PRETENDED that bad things do not happen to our programs. But bad things happen to good programs all the time: a network connection may be down or the printer may run out of paper, for instance. It is not your fault as the programmer when this happens, but you also cannot blame the user if your program crashes because the network goes down! At the very least, your program must not end abruptly when these kinds of things happen. Your program must: •Log or somehow notify the user of the problem. Let the user save his or her work if appropriate. Let the user gracefully exit the program if necessary. This is not always easy. The code to open a network connection is usually not attached to the objects whose state you need to maintain. You often need some way to transfer control as well as to inform other objects what happened so they can deal with the situation.

The idea is that VB .NET finally supports structured exception handling (or simply, exception handling) for dealing with common errors. In this chapter, we not only show you the syntax used to add exception handling to a VB .NET application, but we show you the benefits of using it for error handling. For example, with exception handling, even the more or less socially acceptable use of the GoTo we showed you back in Chapter 3 is no longer necessary. But because power always comes at a cost, we also alert you to the gotchas that you will encounter when using exception handling. NOTEFor readers coming from earlier versions of VB, using the older On Error syntax is still possible. We think, however, that it would be almost foolish to continue using it for new programs. We feel strongly that is has taken far too long for VB to lose an archaic way of treating errors that goes back to the early days of computing! (You also cannot mix the two methods in the same procedure.)

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