Page 336 - Microsoft Office Excel 2003 Programming Inside Out

Microsoft Office Excel 2003 Programming Inside Out

Defining Events
Events can be very useful in a class, but you can’t assume that everyone that uses your class
will actually use the events. Therefore, if you decide to use events in your class, you need to
ensure that the class will continue to function if the user doesn’t respond to any of the events.
The Event statement is used to define an event. For all practical purposes, this is effectively a
subroutine statement minus the code. This definition is necessary because it identifies the
parameters that will be passed to the event. The event definition is used by the by the Visual
Basic compiler to ensure that the number of parameters and the type of the parameters
match the definition.

Note Although you can specify nearly any type of parameter you can use in a subroutine,
events can’t have named arguments, optional parameters, or ParamArray arguments.
Chapter 14
A sample event definition might look like this:

Event DiscountError (value As Currency, Msg As String)

Within the class, you would use a RaiseEvent statement to trigger an event in the user program.
Following the RaiseEvent statement name is the name of the event, followed by a list of values
that will be passed to the user program. For example, this statement passes two values back to
the calling program.

RaiseEvent DiscountError(discount, “Illegal discount amount. “)

To use events in an application, the WithEvents keyword must be included when the object is
defined. Without the WithEvents keyword, all events will be ignored. The following statement
shows how you would declare an object with events:

Dim WithEvents MyObject As GardenCompany

Defining Private Variables, Subroutines, and Functions

Although you need not mark your subroutines or functions as Private in a class, you should
note that without the Private keyword, any subroutine or function will default to Public. In
many situations, this might not be a serious problem, especially if you are the one person
using the class. However, if you plan to share your class with others, you might find them
relying on a routine where you accidentally omitted the Private keyword, which means you
can’t change the definition of the routine without impacting all the programs that use it.

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Part 4: Advanced VBA