按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
as “File XYZ is not present and thus I cannot continue。” Upon seeing such an error message；
another developer would know that he should check whether the file is actually there。
This chapter explains exceptions； as errors in an application are technically known； and how
to handle them。 We’ll begin with an overview of how exceptions fit into the structure of a program。
Understanding Errors； Exceptions； and
Exception Handling
An error is when something is not right and could be the result of incorrect data or an incorrect
calculation。 However； the CLR does not understand errors； it understands only exceptions。
For example； if an error is caused by multiplying two numbers rather than adding them；
your program will continue to function； but will produce the wrong results。 A similar error occurs
when a user enters the wrong data—the answer will be wrong； but the program will still run。
In the case of a serious problem that is beyond the control of the user or that threatens to crash
your program； the CLR steps in and treats this as an exception。 This interrupts the program and
allows your code to deal with the problem； rather than letting the program crash。 （Some would
argue that an exception will not halt your entire program； but a thread of your program； while
115
…………………………………………………………Page 138……………………………………………………………
116 CH AP T E R 5 ■ L E A R N IN G AB OU T V I SU A L B AS IC E X C E P TI ON H AN D L IN G
this is technically true； it’s not an important distinction for this introduction to exceptions。）
This is called exception handling。
To understand how code organization affects exception handling； think of an application
as a large corporation。 A corporation has a chief executive officer （CEO）； then first…level managers；
then mid…level managers。 Corporations and management within corporations understand
that to get anything done； they must develop a game plan and then carry out the game plan。
The CEO or highest level manager will know about the overall game plan。 The pany is orga
nized so that lower…level managers and workers can carry out discrete tasks as defined in the
game plan。 In other words； these organizational units carry out the implementation of the plan。
Translating this to software； your code contains two types of methods： methods that orga
nize the functionality and methods that implement the functionality。 You create organizational
code so that you have the ability to separate each task from the others。 The work of one does
not affect the other； and thus the code has been modularized。
■Note As with corporation reorganization； management code is constantly being reorganized to fix bugs
and implement new pieces of functionality。 For example； you might reorganize code so that it bees more
streamlined and efficient。
Now we need to put exceptions into the picture。 Errors occur when something does not go
according to plan。 In the management hierarchy； if an error occurs； the CEO is not automati
cally told。 For example； if the pany ran out of staples； most likely the CEO would not want
to hear about it。 If； however； one of the manufacturing plants fell off the side of a cliff； then the
CEO would definitely want to know。 The information about the error is passed up the chain of
mand just as far as it needs to go。
Relating this to the different types of methods； in your hierarchical code； the implementa
tion code is responsible for making sure all errors are reported； and the organizational code is
responsible for either fixing the error or propagating it to a piece of code that is higher in the
hierarchy。 The higher piece of code should either fix the problem or delegate it again to a higher
piece of code； depending on the seriousness of the error。
For the remainder of this chapter； we’ll look at the various ways of dealing with exceptions。
The idea is to give you practical solutions that you can use without getting too bogged down in
theoretical what…ifs。
When working with exceptions； it is often useful to run the application in Visual Basic’s
debugger； so let’s start there。
Running the Debugger
The Visual Basic debugger runs the application； but adds the ability to monitor what the appli
cation is doing。 To start the debugger； select Debug Start Debugging from the menu bar or
press F5。
Your application will run as normal； but Visual Basic Express’s layout will change slightly。
From the perspective of the application； you will not see a difference in its behavior。 The appli
cation executes in the same way； regardless of whether it is being run or being debugged。 To
stop debugging； just close your application normally。
…………………………………………………………Page 139……………………………………………………………
CH AP T E R 5 ■ L E AR N IN G AB O U T V I SU A L B AS IC E X CE PT I ON HA N D L IN G 117
■Tip You may find it useful to display the Locals and Call Stack panes； which allow you to see what is going
on in the application。 To do so； start debugging and select Debug Windows； and then choose the window
（Call Stack or Locals）。
You can also start the debugger at a certain point in your code by setting a breakpoint； as
shown in Figure 5…1。 When Visual Basic reaches this breakpoint； it goes into debug mode； this
mode is different from applicati