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
Protect
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.
Debug.Print 1 / 0 but when I get a pop-up window about the error and click end, the Class_Terminate procedure doesn't fire. So worksheet protection is only restored if the procedure runs to completion through error handling or by clicking "Debug" and altering code to not produce an error. Is this expected? –
Dislike 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 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.
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.
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:)
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.
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.
As there is a lot code overhead in using classes in VBA I think a class has to provide more benefit than in other languages:
So this are things to consider before using a class instead of functions:
There is no class-inheritance in vba. So prepare to copy some code when you do similar small things in different classes. This happens especially when you want to work with interfaces and want to implement one interfaces in different classes.
There are no built in constructors in vba-classes. In my case I create a extra function like below to simulate this. But of curse, this is overhead too and can be ignored by the one how uses the class. Plus: As its not possible to use different functions with the same name but different parameters, you have to use different names for your "constructor"-functions. Also the functions lead to an extra debug-step which can be quite annoying.
Public Function MyClass(ByVal someInit As Boolean) As MyClassClass
Set MyClass = New MyClassClass
Call MyClass.Init(someInit)
End Function
The development environment does not provide a "goto definition" for class-names. This can be quite annoying, especially when using classes with interfaces, because you always have to use the module-explorer to jump to the class code.
object-variables are used different to other variable-types in different places. So you have to use a extra "Set" to assign a object
Set varName = new ClassName
if you want to use properties with objects this is done by a different setter. You have to use "set" instead of "let"
If you implement an interface in vba the function-name is named "InterfaceName_functionName" and defined as private. So you can use the interface function only when you cast the Variable to the Interface. If you want to use the function with the original class, you have to create an extra "public" function which only calls the interface function (see below). This creates an extra debug-step too.
'content of class-module: MyClass
implements IMyInterface
private sub IMyInterface_SomeFunction()
'This can only be called if you got an object of type "IMyInterface"
end function
private sub IMyInterface_SomeFunction()
'You need this to call the function when having an object of the type "MyClass"
Call IMyInterface_SomeFunction()
end function
This means:
Examples of classes I created which I found to be useful:
Examples of classes I created which I dont found to be useful:
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.
In VBA, I prefer classes to modules when:
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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