Sams Teach Yourself Visual Basic 6 in 24 Hours

By Greg Perry

• Table of Contents
  • Copyright
  • About the Author
  • Acknowledgments
  • Introduction
  • Who Should Read This Book
  • What This Book Will Do for You
  • Can This Book Really Teach Visual Basic in 24 Hours?
  • What You Need
  • Files on the Visual Basic Distribution CD-ROM
  • Conventions Used in This Book
  • Enough! Time Is Ticking!
  • Part I: Introducing Visual Basic
  • Hour 1. Visual Basic at Work
  • Hour 2.Analyzing Visual Basic Programs
  • Hour 3.Controls and Properties
  • Hour 4.Examining Labels, Buttons, and Text Boxes
  • Part II: Coding the Details
  • Hour 5.Putting Code into Visual Basic
  • Hour 6.Message and Input Boxes
  • Hour 7.Making Decisions
  • Hour 8.Visual Basic Looping
  • Part III:Putting Code to Work
  • Hour 9.Combining Code and Controls
  • Hour 10.List Boxes and Data Lists
  • Hour 11.Additional Controls
  • Hour 12.Dialog Box Basics
  • Part IV:Programming with Data
  • Hour 13.Modular Programming
  • Hour 14.Built-In Functions Save Time
  • Hour 15.Visual Basic Database Basics
  • Hour 16.Printing with Visual Basic
  • Part V:Sprucing Up Programs
  • Hour 17.Menus and Visual Basic
  • Hour 18.The Graphic Image Controls
  • Hour 19.Toolbars and More Graphics
  • Hour 20.Writing Correct Applications
    • Kinds of Errors
    • The Debugger
    • Setting Breakpoints
    • Stepping Through Code
    • The Call Stack Shows Where You've Been
    • The Immediate Window
    • Summary
    • Q&A
    • Workshop
  • Part VI:Advancing Visual Basic Applications
  • Hour 21.Visual Basic and ActiveX
  • Hour 22.Object Basics
  • Hour 23.Distributing Your Applications
  • Hour 24.Online Visual Basic
  • Part VII:Appendixes
  • Appendix A.Operator Precedence
  • Appendix B.Answers
  • Appendix C.Using the CD-ROM

The Call Stack Shows Where You've Been

At any point during the debugging session, you can click the Debug toolbar button's Call Stack button to display the Call Stack window (see Figure 20.9).

Figure 20.9 The Call Stack window lists all called procedures.

The Call Stack window shows where your program execution has traveled. In addition, any non-Visual Basic routines, such as Windows routines that sometimes take over, appear in the Call Stack window. The call stack keeps a running list of all procedures executed, even if the same procedure executes multiple times.

Suppose that a variable contains an incorrect value but you're not exactly sure where the error is occurring. You could set a breakpoint at every line of code that changes the variable. When you run the program, you'll look at the contents of that variable before and after each breakpoint's line of execution. If the first breakpoint seems to initialize the variable properly, you don't have to single-step through the code until the next break-point is reached. Instead of single-stepping, you can select Run | Continue or press F5 to return the execution to its normal runtime (and real-time) mode. When Visual Basic reaches the next breakpoint, the code halts, and you can continue to examine the variable.

At a breakpoint, you can add not only variables but Watches window expressions as well. Suppose that a variable is to maintain a count of customers, but somewhere in your code a negative value appears in the variable. You can debug this problem by adding a watch expression such as intCustCnt < 0 to the Watches window. To do this, right-click the window and select Add Watch to display the Add Watch dialog box (see Figure 20.10). Click the window's Break When Value Is True option button. You can then run the programs, and Visual Basic enters break mode at any line that causes the variable to become negative.

Figure 20.10 Watching for expressions as well as variables and controls.

The breakpoints and watch dialog boxes that you can request while debugging your code give you tremendous power in analyzing variables and watching for specific results. You can look at the contents of variables and controls to make sure that data is being initialized the way you expect. Also, the Add Watch dialog box lets you set up expressions that Visual Basic watches for during the program's execution. If the values of those expressions ever become true or change, Visual Basic halts the code at that line and lets you analyze values using the Watches window.

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