Visual Basic 6 Black Book
(Publisher: The Coriolis Group)
Author(s): Steven Holzner
ISBN: 1576102831
Publication Date: 08/01/98

Chapter 29
Error Handling And Debugging

If you need an immediate solution to:

Writing Error Handlers

Using On Error GoTo Label

Using On Error GoTo line#

Using On Error Resume Next

Using On Error GoTo 0

Using Resume In Error Handlers

Using Resume Label In Error Handlers

Using Resume line# In Error Handlers

Using Resume Next In Error Handlers

Getting An Error’s Error Code

Getting An Error’s Description

Determining An Error’s Source Object

Handling Errors In DLLs: The LastDLLError Property

Creating An Intentional (User-Defined) Error

Nested Error Handling

Creating An Error Object Directly In Visual Basic

Trappable Cancel Errors In Common Dialogs

Debugging In Visual Basic

Setting Debugging Breakpoints

Single-Stepping While Debugging

Examining Variables And Expressions

Adding Debug Watch Windows

Using The Immediate Window While Debugging

Clearing All Debugging Breakpoints

Executing Code Up To The Cursor While Debugging

Skipping Over Statements While Debugging

In Depth

This is our chapter on runtime errors and bugs. With a process called trapping, Visual Basic lets you catch many runtime errors, and we’ll see how to do that here. And when we catch a runtime error, we’ll see how to recover from that error without crashing the program. Just about every Visual Basic programmer is familiar with bugs—they’re those annoying logic errors that occur when what the computer does doesn’t appear to be the same as what you asked it to do. We’ll ferret out bugs in this chapter with the Visual Basic debugger.

We’ll also see how to get and interpret error codes—including translating them to English, how to write error handlers, how to use the Resume statement to continue program execution, and quite a few other powerful topics. Handling runtime errors is usually a part of any commercially released Visual Basic program because Visual Basic handles runtime errors by displaying information only useful to the programmer. Visual Basic lets you handle runtime errors by trapping them with special code, and these errors are referred to as trappable errors. You’ll find a list of the Visual Basic trappable errors in Table 29.1.

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TIP:  Getting “out of memory” errors has driven more than one programmer to distraction, How can I be out of memory? I have 512MB of RAM! In fact, Microsoft sometimes treats this error as a generic error, and programs can issue this error when the actual error cause is unknown.

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With regard to debugging, Visual Basic provides programmers with a set of tools that is hard to beat. You can debug your programs interactively, working through your code line by line as the program runs. This powerful technique lets you work behind the scenes in a way that is invaluable to finding out what’s going wrong. You can also specify where to begin debugging a program with breakpoints, which are lines of code that you tag to make the program halt and debugging start.

In this chapter, we’ll see how to set and use breakpoints, execute code line by line, execute code up to a specified line, watch variables as they’re changing, and more. Taking care of your program’s bugs before you release it is important—the user might be able to handle runtime errors with the aid of our error handlers, but not errors in program logic.