Stephens' C# Programming with Visual Studio® 2010 24-Hour Trainer

Handling Errors

The best way to handle errors is to not give the user the ability to make them in the first place. For

example, suppose a program can take purchase orders for between 1 and 100 reams of paper. If

the program lets you specify the quantity by using a

NumericUpDown control with Minimum = 1

and

Maximum = 100, you cannot accidentally enter invalid values like –5 or 10,000.

Sometimes, however, it’s hard to build an interface that protects against all possible errors. For

example, if the user needs to type in a numeric value, you need to worry about invalid inputs

such as 1.2.3 and ten. If you write a program that works with files, you can’t always be sure the

file will be available when you need it. For example, it might be on a CD or floppy disk that has

been removed, or it might be locked by another program.

In this lesson, you learn how to deal with these kinds of unexpected errors. You learn how to

protect against invalid values, unavailable files, and other problems that are difficult or impos-

sible to predict in advance.

ERRORS AND EXCEPTIONS

An error is a mistake. It occurs when the program does something incorrect. Sometimes an

error is a bug, for example, if the code just doesn’t do the right thing.

Sometimes an error is caused by circumstances outside of the program’s control. If the program

expects the user to enter a numeric value in a textbox but the user types 1.