Visual Basic.NET How to Program (PDF)

458 Exception Handling Chapter 11
fore, the exception is not caught in method ThrowExceptionWithoutCatch. Normal
program control cannot continue until the exception is caught and processed. Thus, the
CLR terminates ThrowExceptionWithoutCatch, and program control returns to
Main. Before control returns to Main, the Finally block (lines 110–112) executes and
outputs a message. At this point, program control returns to Main—any statements
appearing after the Finally block (e.g., line 117) do not execute. In this example, such
statements could cause logic errors, because the exception thrown in lines 106–107 is not
caught. In Main, the Catch handler in lines 30–32 catches the exception and displays a
message indicating that the exception was caught in Main.
Common Programming Error 11.6
The argument of a Throw—an exception object—must be of class Exception or one of its
derived classes. 11.6
Lines 42–50 of Main define a Try block in which Main invokes method Throw-
ExceptionCatchRethrow (lines 121–149). The Try block enables Main to catch
any exceptions thrown by ThrowExceptionCatchRethrow. The Try block in lines
124–145 of ThrowExceptionCatchRethrow begins by outputting a message. Next,
the Try block throws an Exception (lines 127–128). The Try block expires immediately,
and program control continues at the first Catch (lines 131–136) following the Try
block. In this example, the type thrown (Exception) matches the type specified in the
Catch, so lines 132–133 outputs a message indicating where the exception occurred. Line
136 uses the Throw statement to rethrow the exception. This indicates that the Catch
handler performed partial processing of the exception and now is passing the exception
back to the calling method (in this case, Main) for further processing. Note that the argument
to the Throw statement is the reference to the exception that was caught. When
rethrowing the original exception, you also can use the statement
Throw
with no argument. Section 11.6 demonstrates using a Throw statement with an argument
from a Catch handler. After an exception is caught, such a Throw statement enables programmers
to create an exception object then throw a different type of exception from the
Catch handler. Class-library designers often do this to customize the exception types thrown
from methods in their class libraries or to provide additional debugging information.
Software Engineering Observation 11.7
Before rethrowing an exception to a calling method, the method that rethrows the exception
should release any resources it acquired before the exception occurred. 7
11.7
Software Engineering Observation 11.8
Whenever possible, a method should handle exceptions that are thrown in that method, rather
than passing the exceptions to another region of the program. 11.8
The exception handling in method ThrowExceptionCatchRethrow does not
complete, because the program cannot run code in the Catch handler placed after the
invocation of the Throw statement (line 136). Therefore, method Throw-
ExceptionCatchRethrow terminates and returns control to Main. Once again, the
7. “Best Practices for Handling Exceptions [Visual Basic].”