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Chapter 8: Object-Oriented Programming
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Adding Error Handling to Classes
Classes have to provide exceptionally robust error handling so that they’re
reliable. The earlier “Defining methods” section shows one type of error 
handling that you can add to a class. Keeping track of property interactions
before you perform a task such as displaying the message box is important.
You can add other forms of error handling to your class. Tracking actual
input values is another good form of error handling when you can’t use enu-
merated constants. It’s also important to monitor property values based on
other input values. Here’s an example of code (see Listing 8-7) that monitors
the 
DefaultButton
property based on the number of buttons that the mes-
sage box has. The code ensures that the default button is a button that actu-
ally exists on the message box.
Listing 8-7
Providing Error Handling in an Object
Public Static Property Let DefaultButton(Value As
DefaultButtonTypes)
‘ Change the value of the message icon based on the
‘ input value.
Select Case Value
Case Button_1
UseDefault = vbDefaultButton1
Case Button_2
If ((UseButtons = vbOKOnly) And _
(Not UseHelpButton)) Then
‘ Can’t have a single button setting if
‘ there is no Help button, so raise an
‘ error.
Err.Raise vbObjectError + 2, _
“SpecialMsg.DefaultButton”, _
“The selected default button value is “ +
_
“incorrect. Choose a default button “ + _
“that matches the message box settings.”
Else
‘ Set the default button value.
UseDefault = vbDefaultButton2
End If
Case Button_3
If ((UseButtons = vbOKOnly) Or _
(((UseButtons = vbOKCancel) Or _
(UseButtons = vbRetryCancel) Or _
(UseButtons = vbYesNo)) And _
(Not UseHelpButton))) Then
‘ This setting doesn’t support a single
‘ button option at all. It also doesn’t
‘ support any of the double button options
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