Visual Basic.NET How to Program (PDF)

1304 Visual Studio .NET Debugger Appendix D
Testing and Debugging Tip D.2
After fixing one error, recompile your program. You may observe that the number of overall
errors perceived by the compiler is significantly reduced. 24.0
Debugging is the process of finding and correcting logic errors in applications. Logic
errors are more subtle than syntax errors because the program compiles successfully, but
does not run as expected. Logic errors are often difficult to debug because the programmer
cannot see the code as it is executing. Some programmers attempt to debug programs using
message boxes or Console.WriteLine statements. For example, the programmer
might print the value of a variable when the variable’s value changes to determine if it is
being set correctly. This method is cumbersome, because programmers must write a line of
code wherever they suspect may be a problem. Once the program has been debugged, the
programmer must remove these printing statements.
Debuggers provide a set of tools that allow the programmer to analyze a program while
it is running. These tools allow the programmer to suspend program execution, examine and
set variables, call procedures without having to modify the program and much more. In this
appendix, we introduce the Visual Studio .NET debugger and several of its debugging tools.
[Note: A program must successfully compile before it can be used in the debugger.]
D.2 Breakpoints
Breakpoints are a simple but powerful debugging tool. A breakpoint is a marker that can be
set at any executable line of code. When a program reaches a breakpoint, execution pauses,
allowing the programmer to examine the state of the program and ensure that everything is
working properly. We use the following program (Fig. D.2) to demonstrate debugging a
loop using the features of the Visual Studio .NET debugger. This program is designed to
output the value of ten factorial (10!), but contains two logic errors—the first iteration of
the loop multiplies x by 10 instead of 9, and the result of the factorial calculation 0.
1 ' Fig. D.2: DebugExample.vb
2 ' Sample program to debug.
3
4 Module modDebug
5
6 Sub Main()
7 Dim x As Integer = 10
8 Dim i As Integer
9
10 Console.Write("The value of " & x & " factorial is: ")
11
12 ' loop to determine x factorial, contains logic error
13 For i = x To 0 Step -1
14 x *= i
15 Next
16
17 Console.WriteLine(x)
18 End Sub ' Main
19
20 End Module ' modDebug
Fig. D.2 Debug sample program (part 1 of 2).