Creating and Coding a Class: a Simple Worked Example
For this page, we're going to look at a simple, cut-down example of the input box introduced in the previous part of this blog:
Our input box class will have:
In VBA terms,
we say that the class (which we'll call clsLetterGuess) exposes
the property and method shown above.
Creating the Class Module
To create a new class, just insert a class module:
Right-click in
Project Explorer and choose to insert a Class Module.
You now need to give your class a name (initially it will be called
Class1) in the Properties window:
You can press F4
to bring up the Properties
window, and overtype the Name
property.
You might at this stage like to experience the truly memorable moment when your
class first appears in Intellisense, by creating a new
procedure in a separate module as follows:
Your new class appears in the list of objects available!
Creating a
Simple Property
There are two types of property that you can create in VBA. We'll deal
with the more complicated read-write properties later in this blog, but you can
create a simple property just by adding a variable to the top of your class:
Option Explicit
Public LetterGuessed As String
It's vital that this variable is Public, because you want to
be able to get at its value from outside the class:
This wouldn't happen if you made the variable
Private, as its value wouldn't be exposed by the class to the outside world.
Creating a Method
A method is just a subroutine, but again we'll make it Public
(the default), as we want to expose the method from the class:
Option Explicit
Public LetterGuessed As String
Private Const Letters As String = _
"abcdefghijklmnopqrstuvwxyz"
Sub StartGuess()
Dim Letter As String
Const MaxGuesses As Integer = 3
Dim GuessNumber As Integer
GuessNumber = 1
LetterGuessed = ""
Do Until Len(LetterGuessed) > 0 Or GuessNumber > MaxGuesses
Letter = InputBox("Think of a letter", _
"Guess", "Type letter here")
If Len(Letter) <> 1 Then
MsgBox "You must type in one (and only one) letter"
ElseIf InStr(1, Letters, LCase(Letter)) <= 0 Then
MsgBox "Not a valid letter"
Else
LetterGuessed = Letter
End If
GuessNumber = GuessNumber + 1
Loop
End Sub
Note that we've added a constant called Letters for the
class to hold the alphabet (the allowed letters). At the end of running
the method StartGuess above, LetterGuessed will either hold
a valid letter guessed, or an empty string.
Consuming a Class
To consume your class (ie to create an object based on it), we could create
the following procedure in a separate module:
Sub RequestLetter()
Dim Guess As New clsLetterGuess
Guess.StartGuess
If Len(Guess.LetterGuessed) = 0 Then
MsgBox "No letter guessed"
Else
MsgBox "You guessed " & Guess.LetterGuessed
End If
Set Guess = Nothing
End Sub
Note that we can only refer to public properties and methods of the class:
You can only see properties and methods (collectively called members) which are publicly exposed.
Stop snickering at the back - yes, you really can talk about class members being
publicly exposed!
Debugging a Class
When you run the code to consume a class, you can often get spurious errors:
Here our code has crashed - I wonder where?
When you click on the Debug button above, you see
the offending line:
The code crashed on this line.
To find out what's really going on, you'll need to set a breakpoint within the
class:
The real culprit: we're trying to store a letter in an integer variable, but you can only find this out by stepping throught the code.
An alternative approach is to change your VBA settings to allow code to break
within a class module: from the VBA menu select Tools --> Options,
click on the General tab and select the obvious error trapping
option.
Conclusion: Was it Worth It?
A class is like a swan: from the outside it looks serene and simple, but
there's a lot of furious paddling going on underneath the water. The
following code is nice and straightforward:
Dim Guess As New clsLetterGuess
Guess.StartGuess
If Len(Guess.LetterGuessed) = 0 Then
MsgBox "No letter guessed"
Else
MsgBox "You guessed " & Guess.LetterGuessed
End If
The price you have to pay, however, is in writing and testing the properties
and methods in the clsLetterGuess class.
Classes are ideally suited to a situation where you have a small team of
expert programmers to create classes, and a larger team of less expert
programmers who will want to use them over and over again.
