Visual Basic.NET How to Program (PDF)

460 Exception Handling Chapter 11
Testing and Debugging Tip 11.7
A stack trace shows the complete method-call stack at the time an exception occurred. This
enables the programmer to view the series of method calls that led to the exception. Information
in the stack trace includes the names of the methods on the call stack at the time of the
exception, names of the classes in which those methods are defined, names of the namespaces
in which those classes are defined. The stack trace also includes line numbers; the first line
number indicates the throw point, and subsequent line numbers indicate the locations from
which the methods in the stack trace were called. 11.7
Another property used frequently by class-library programmers is InnerException.
Typically, programmers use this property to “wrap” exception objects caught in
their code so that they then can throw new exception types that are specific to their libraries.
For example, a programmer implementing an accounting system might have some accountnumber
processing code in which account numbers are input as Strings, but represented
as Integers in the code. Recall, a program can convert Strings to Integer values
with Convert.ToInt32, which throws a FormatException when it encounters an
invalid number format. When an invalid account-number format occurs, the accountingsystem
programmer might wish employ a different error message than the default message
supplied by FormatException or might wish to indicate a new exception type, such as
InvalidAccountNumberFormatException. In these cases, the programmer
would provide code to catch the FormatException and then would create an Exception
object in the Catch handler, passing the original exception as one of the constructor
arguments. The original exception object becomes the InnerException of the new
exception object. When an InvalidAccountNumberFormatException occurs in
code that uses the accounting-system library, the Catch block that catches the exception
can obtain a reference to the original exception via property InnerException. Thus,
the exception indicates both that the user specified an invalid account number and that the
particular problem was an invalid number format.
Class Exception provides other properties, including HelpLink, Source and
TargetSite. Property HelpLink specifies the location of the help file that describes
the problem that occurred. This property is Nothing if no such file exists. Property
Source specifies the name of the application where the exception occurred. Property
TargetSite specifies the method where the exception originated.
Our next example (Fig. 11.3) demonstrates properties Message, StackTrace and
InnerException and method ToString of class Exception. In addition, this
example introduces stack unwinding, which is the process of attempting to locate an appropriate
Catch handler for an uncaught exception. As we discuss this example, we keep
track of the methods on the call stack so that we can discuss property StackTrace and
the stack-unwinding mechanism.
Program execution begins with the invocation of Main, which becomes the first
method on the method call stack. Line 13 of the Try block in Main invokes Method1
(defined in lines 37–39), which becomes the second method on the stack. If Method1
throws an exception, the Catch handler in lines 17–30 handles the exception and outputs
information about the exception that occurred. Line 38 of Method1 invokes Method2
(lines 42–44), which becomes the third method on the stack. Then, line 43 of Method2
invokes Method3 (lines 47–61) which becomes the fourth method on the stack.