Programming Visual Basic .NET, Second Edition

Chapter 11. Exceptions

VB.NET handles errors and abnormal conditions with exceptions. An exception is an object that encapsulates information about an unusual program oCcurrence, such as running out of memory or losing a network connection.

It is important to distinguish exceptions from bugs and errors. A bug is a programmer mistake that should be fixed before the code is shipped. An exception is not the result of a programmer mistake (though such mistakes can also raise exceptions). Rather, exceptions are raised as a result of predictable but unpreventable problems that arise while your program is running (e.g., a network connection is dropped or you run out of disk space).

An error is caused by user action. For example, the user might enter a number where a letter is expected. Once again, an error might cause an exception, but you can prevent that by implementing code to validate user input. Whenever possible, user errors should be anticipated and prevented.

Even if you remove all bugs and anticipate all user errors, you will still run into unavoidable problems, such as running out of memory or attempting to open a file that no longer exists. These are exceptions. You cannot prevent exceptions, but you can handle them so that they do not bring down your program.

When your program encounters an exceptional circumstance, such as running out of memory, it throws (or “raises”) an exception. You might throw an exception in your own methods (for example, if you realize that an ...