When to use a Class in VBA?

When is it appropriate to use a class in Visual Basic for Applications (VBA)?

I'm assuming the accelerated development and reduction of introducing bugs is a common benefit for most languages that support OOP. But with VBA, is there a specific criterion?


It depends on who's going to develop and maintain the code. Typical "Power User" macro writers hacking small ad-hoc apps may well be confused by using classes. But for serious development, the reasons to use classes are the same as in other languages. You have the same restrictions as VB6 - no inheritance - but you can have polymorphism by using interfaces.

A good use of classes is to represent entities, and collections of entities. For example, I often see VBA code that copies an Excel range into a two-dimensional array, then manipulates the two dimensional array with code like:

Total = 0
For i = 0 To NumRows-1
    Total = Total + (OrderArray(i,1) * OrderArray(i,3))
Next i

It's more readable to copy the range into a collection of objects with appropriately-named properties, something like:

Total = 0
For Each objOrder in colOrders
    Total = Total + objOrder.Quantity * objOrder.Price
Next i

Another example is to use classes to implement the RAII design pattern (google for it). For example, one thing I may need to do is to unprotect a worksheet, do some manipulations, then protect it again. Using a class ensures that the worksheet will always be protected again even if an error occurs in your code:

--- WorksheetProtector class module ---

Private m_objWorksheet As Worksheet
Private m_sPassword As String

Public Sub Unprotect(Worksheet As Worksheet, Password As String)
    ' Nothing to do if we didn't define a password for the worksheet
    If Len(Password) = 0 Then Exit Sub

    ' If the worksheet is already unprotected, nothing to do
    If Not Worksheet.ProtectContents Then Exit Sub

    ' Unprotect the worksheet
    Worksheet.Unprotect Password

    ' Remember the worksheet and password so we can protect again
    Set m_objWorksheet = Worksheet
    m_sPassword = Password
End Sub

Public Sub Protect()
    ' Protects the worksheet with the same password used to unprotect it
    If m_objWorksheet Is Nothing Then Exit Sub
    If Len(m_sPassword) = 0 Then Exit Sub

    ' If the worksheet is already protected, nothing to do
    If m_objWorksheet.ProtectContents Then Exit Sub

    m_objWorksheet.Protect m_sPassword
    Set m_objWorksheet = Nothing
    m_sPassword = ""
End Sub

Private Sub Class_Terminate()
    ' Reprotect the worksheet when this object goes out of scope
    On Error Resume Next
End Sub

You can then use this to simplify your code:

Public Sub DoSomething()
   Dim objWorksheetProtector as WorksheetProtector
   Set objWorksheetProtector = New WorksheetProtector
   objWorksheetProtector.Unprotect myWorksheet, myPassword

   ... manipulate myWorksheet - may raise an error

End Sub 

When this Sub exits, objWorksheetProtector goes out of scope, and the worksheet is protected again.

I think the criteria is the same as other languages

If you need to tie together several pieces of data and some methods and also specifically handle what happens when the object is created/terminated, classes are ideal

say if you have a few procedures which fire when you open a form and one of them is taking a long time, you might decide you want to time each stage......

You could create a stopwatch class with methods for the obvious functions for starting and stopping, you could then add a function to retrieve the time so far and report it in a text file, using an argument representing the name of the process being timed. You could write logic to log only the slowest performances for investigation.

You could then add a progress bar object with methods to open and close it and to display the names of the current action, along with times in ms and probable time remaining based on previous stored reports etc

Another example might be if you dont like Access's user group rubbish, you can create your own User class with methods for loging in and out and features for group-level user access control/auditing/logging certain actions/tracking errors etc

Of course you could do this using a set of unrelated methods and lots of variable passing, but to have it all encapsulated in a class just seems better to me.

You do sooner or later come near to the limits of VBA, but its quite a powerful language and if your company ties you to it you can actually get some good, complex solutions out of it.

Classes are extremely useful when dealing with the more complex API functions, and particularly when they require a data structure.

For example, the GetOpenFileName() and GetSaveFileName() functions take an OPENFILENAME stucture with many members. you might not need to take advantage of all of them but they are there and should be initialized.

I like to wrap the structure (UDT) and the API function declarations into a CfileDialog class. The Class_Initialize event sets up the default values of the structure's members, so that when I use the class, I only need to set the members I want to change (through Property procedures). Flag constants are implemented as an Enum. So, for example, to choose a spreadsheet to open, my code might look like this:

Dim strFileName As String
Dim dlgXLS As New CFileDialog

With dlgXLS
  .Title = "Choose a Spreadsheet"
  .Filter = "Excel (*.xls)|*.xls|All Files (*.*)|*.*"
  .Flags = ofnFileMustExist OR ofnExplorer

  If OpenFileDialog() Then
    strFileName = .FileName
  End If
End With
Set dlgXLS = Nothing

The class sets the default directory to My Documents, though if I wanted to I could change it with the InitDir property.

This is just one example of how a class can be hugely beneficial in a VBA application.

I wouldn't say there's a specific criterion, but I've never really found a useful place to use Classes in VBA code. In my mind it's so tied to the existing models around the Office apps that adding additional abstraction outside of that object model just confuses things.

That's not to say one couldn't find a useful place for a class in VBA, or do perfectly useful things using a class, just that I've never found them useful in that environment.

You can also reuse VBA code without using actual classes. For example, if you have a called, VBACode. You can access any function or sub in any module with the following syntax:

VBCode.mysub(param1, param2)

If you create a reference to a template/doc (as you would a dll), you can reference code from other projects in the same way.

Developing software, even with Microsoft Access, using Object Oriented Programming is generally a good practice. It will allow for scalability in the future by allowing objects to be loosely coupled, along with a number of advantages. This basically means that the objects in your system will be less dependent on each other, so refactoring becomes a lot easier. You can achieve this is Access using Class Modules. The downside is that you cannot perform Class Inheritance or Polymorphism in VBA. In the end, there's no hard and fast rule about using classes, just best practices. But keep in mind that as your application grows, the easier it is to maintain using classes.

For data recursion (a.k.a. BOM handling), a custom class is critically helpful and I think sometimes indispensable. You can make a recursive function without a class module, but a lot of data issues can't be addressed effectively.

(I don't know why people aren't out peddling BOM library-sets for VBA. Maybe the XML tools have made a difference.)

Multiple form instances is the common application of a class (many automation problems are otherwise unsolvable), I assume the question is about custom classes.

I use classes when I need to do something and a class will do it best:) For instance if you need to respond to (or intercept) events, then you need a class. Some people hate UDTs (user defined types) but I like them, so I use them if I want plain-english self-documenting code. Pharmacy.NCPDP being a lot easier to read then strPhrmNum :) But a UDT is limited, so say I want to be able to set Pharmacy.NCPDP and have all the other properties populate. And I also want make it so you can't accidentally alter the data. Then I need a class, because you don't have readonly properties in a UDT, etc.

Another consideration is just simple readability. If you are doing complex data structures, it's often beneficial to know you just need to call Company.Owner.Phone.AreaCode then trying to keep track of where everything is structured. Especially for people who have to maintain that codebase 2 years after you left:)

My own two cents is "Code With Purpose". Don't use a class without a reason. But if you have a reason then do it:)

I use classes if I want to create an self-encapsulated package of code that I will use across many VBA projects that come across for various clients.

You can define a sql wrapper class in access that is more convenient than the recordsets and querydefs. For example if you want to update a table based on a criteria in another related table, you cannot use joins. You could built a vba recorset and querydef to do that however i find it easier with a class. Also, your application can have some concept that need more that 2 tables, it might be better imo to use classes for that. E.g. You application track incidents. Incident have several attributes that will hold in several tables {users and their contacts or profiles, incident description; status tracking; Checklists to help the support officer to reply tonthe incident; Reply ...} . To keep track of all the queries and relationships involved, oop can be helpful. It is a relief to be able to do Incident.Update(xxx) instead of all the coding ...

I don't see why the criteria for VBA would be any different from another language, particularly if you are referring to VB.NET.

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