Microsoft Visual Basic 2010 Step by Step eBook

240 Part II Programming Fundamentals

Retries variable at the top of the form’s program code so that it has scope throughout all

the form’s event procedures. The Retries variable is then incremented and tested in the

Catch code block. The number of retries can be modified by simply changing the “2” in the

statement, as shown here:

If Retries <= 2

Use a variable to track run-time errors

1. In the Code Editor, scroll to the top of the form’s program code, and directly below the
   Public Class Form1 statement, type the following variable declaration:
   Dim Retries As Short = 0
   Retries is declared as a Short integer variable because it won’t contain very big numbers.
   It’s assigned an initial value of 0 so that it resets properly each time the program runs.


2. In the Button1Click event procedure, edit the Try... Catch error handler so that it looks
   like the following code block:
   Try
   PictureBox1.Image =
   System.Drawing.Bitmap.FromFile("d:\fileopen.bmp")
   Catch
   Retries += 1
   If Retries <= 2 Then
   MsgBox("Please insert the disc in drive D!")
   Else
   MsgBox("File Load feature disabled")
   Button1.Enabled = False
   End If
   End Try
   The Try block tests the same file-opening procedure, but this time, if an error occurs,
   the Catch block increments the Retries variable and tests the variable to be sure that
   it’s less than or equal to 2. The number 2 can be changed to allow any number of
   retries—currently it allows only two run-time errors. After two errors, the Else clause
   is executed, and a message box appears indicating that the file-loading feature has
   been disabled. The Check Drive button is then disabled—in other words, dimmed
   and rendered unusable for the remainder of the program.

Tip This revised version of the error handler that you have been building has been renamed

Disc Drive Handler and is stored in the C:\Vb10sbs\Chap09\Disc Drive Handler folder. You

may notice the new project title in the title bar of your message boxes, but otherwise the

project is the same as what you have been experimenting with thus far. (I’ve simply saved the

revised version so that you can open it later if you want .)

3. Click the Start Debugging button to run the program.


4. Remove the CD or DVD from drive D.