Class Modules in Microsoft Access
(aka.emulating Object-Oriented programming)

Making it do something

Introduction

Now we've seen how to create an object with properties, we can make the object do something. A function in a class module that does something is known as a method.

Again, we'll use a simple example to demonstrate the basics.

Instructions

1. Make a copy of our first class module: in the Modules tab, click once on the object clsSimple, select Edit > Copy then Edit > Paste and name the new module clsSimpleMethod

2. Double-click on clsSimpleMethod and edit the code so it reads:

   Option Compare Database
   Option Explicit
   
   Public OrderID As Long
   Public CustomerName As String
   Public Value As Currency
   
   Public Function Tax() As Currency
       Tax = 0.1 * Me.Value
   End Function
   

3. Save and close this module then Edit the module basTest and Insert a new Procedure with the name TestTax

4. Edit the code:

   Public Function TestTax()
       Dim oSim As New clsSimpleMethod
       Dim cValue As Currency
       
       For cValue = 100 To 1000 Step 100
           oSim.Value = cValue
           Debug.Print "Value=" & oSim.Value, "Tax=" & oSim.Tax
       Next
   End Function
   

5. Save, close and run from the Debug Window by typing the word TestTax and pressing Enter.

   This should produce the results:

   testtax
   Value=100     Tax=10
   Value=200     Tax=20
   Value=300     Tax=30
   Value=400     Tax=40
   Value=500     Tax=50
   Value=600     Tax=60
   Value=700     Tax=70
   Value=800     Tax=80
   Value=900     Tax=90
   Value=1000    Tax=100
   

What we've learned

Just adding the code for a public function Tax to our simple object has created a method that calculates the sales tax at a straight 10 per cent of whatever its value is at the time.

Again, it's hardly testing our computer, but it could equally well be as complicated as we like.

What this means

Even with this simple example, the programmer using the object clsSimpleMethod in his VBA code does not need to know any of the messy details of how the tax is calculated. He just uses the expression oSim.Tax in his code.

Meanwhile, from a higher level of program maintenance point of view, if we want to change the rate of tax to, say, 12%, we only have to alter one number in the code in clsSimpleMethod and, then, every time anyone has used the expression oSim.Tax anywhere in the program, the answer will be automatically correct.

You're probably thinking that, so far, this is really no different from using a public function in a normal code module, apart from the auto list prompting. We'll see in later examples why using classes is much more powerful.

A more complicated case

If the tax laws become more complicated so, say, the tax is 10% if the value is $100 or less and 12% if more, we can change the code in our class module clsSimpleMethod as follows.

Public Function Tax() As Currency
    If Me.Value <= 100 Then
        Tax = 0.1 * Me.Value
    Else
        Tax = 0.12 * Me.Value
    End If
End Function

set os = New clsSimpleMethod
os.value = 100
? os.tax
 10 
os.value = 200
? os.tax
 24 

Question: What do you think the word Me means in the expression Me.Value?