Microsoft Visual Basic 2010 Step by Step eBook

Chapter 9 Trapping Errors by Using Structured Error Handling 233

Writing a Disc Drive Error Handler

The problem with the Disc Drive Error program isn’t that it somehow defies the

inherent capabilities of Visual Basic to process errors. We just haven’t specified what

Visual Basic should do when it encounters an exception that it doesn’t know how

to handle. The solution to this problem is to write a Try... Catch code block that

recognizes the error and tells Visual Basic what to do about it. You’ll add this error

handler now.

Use Try... Catch to trap the error

1. Display the Button1_Click event procedure if it isn’t visible in the Code Editor.

You need to add an error handler to the event procedure that’s causing the problems.

As you’ll see in this example, you actually build the Try... Catch code block around the

code that’s the potential source of trouble, protecting the rest of the program from

the run-time errors that it might produce.

2. Modify the event procedure so that the existing FromFile statement fits between Try
   and Catch statements, as shown in the following code block:

Try

PictureBox1.Image = _

System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")

Catch

MsgBox("Please insert the disc in drive D!")

End Try

You don’t need to retype the FromFile statement—just type the Try, Catch, MsgBox,

and End Try statements above and below it. If Visual Studio adds Catch, variable

declaration, or End Try statements in the wrong place, simply delete the statements and

retype them as shown in the book. (The Code Editor tries to be helpful, but its Auto

Complete feature sometimes gets in the way .)

This program code demonstrates the most basic use of a Try... Catch code block. It

places the problematic FromFile statement in a Try code block so that if the program

code produces an error, the statements in the Catch code block are executed. The

Catch code block simply displays a message box asking the user to insert the required

disc in drive D so that the program can continue. This Try... Catch code block contains

no Finally statement, so the error handler ends with the keywords End Try.

Again, if you are using a removable storage device or media associated with a

different drive letter, you would make those changes in the statements that you

just typed.